HESI Entrance Exam Study guide

body planes
body planes
imaginary lines used for reference; they include the median plane, the coronal plane, and the transverse plane.

Median Plane
divides the body into right and left halves. Also called the midsagittal plane.

Planes of the Body

Coronal Plane
divides the body into front and rear sections. Also called the frontal plane.

Frontal plane
divides the body into front and rear sections. Also called the coronal plane.

Horizontal Plane
divides the body into a superior (or upper) and an inferior (or lower) section. Also called the transverse plane.

Median Plane
divides the body into right and left halves. Also called the midsagittal plane.

Midsagittal Plane
divides the body into right and left halves. Also called the median plane.

Transverse Plane
divides the body into a superior (or upper) and an inferior (or lower) section. Also called the horizontal plane.

Direction and Location

Anterior
front side of the body, also known as ventral.

Caudal
in quadrapeds, the tail end [see inferior].

Cranial
Cranial
above or near the head, also known as superior.

Distal
farthest end from the trunk or head.

Dorsal
back side of the body, also known as the posterior.

Inferior
Inferior
below also, toward the feet.

Infra-
prefix meaning below or under.

Lateral
away from the midline.

Medial
toward the midline.

Posterior
back side of the body, also known as the dorsal.

Proximal
closest part nearest the trunk or head.

Superior
above or near the head, also known as cranial.

Supra-
prefix meaning above or over.

Ventral
front side of the body, also known as anterior.

Parts of the Human Skull

Calvarium
includes the brain case.

Cranium
includes the face and the calvarium.

Mandible
the lower jaw.

Skull
Skull
includes both the cranium and mandible.

Bones of the Skull

Ethmoid bone
Ethmoid bone
sieve-like spongy bone located in the anterior part of the floor of the cranium between the orbits. The ethmoid is the principal supporting structure of the nasal cavity.

Frontal bone
forms the forehead, the roofs of the orbits, and most of the anterior part of the cranial floor.

Inferior Nasal Conchae
Inferior Nasal Conchae
one of three scroll-like bones that project from the lateral wall of the nasal cavity. The inferior nasal conchae articulate with the ethmoid, maxilla, lacrimal and paltine bones and form the lower part of the lateral wall of the nasal cavity.

Lacrimal bone
Lacrimal bone
a thin scalelike bone, roughly resembling a fingernail in size and shape, at the anterior part of the medial wall of the orbit, articulating with the frontal and ethmoidal bones and the maxilla and inferior nasal concha. The lacrimal bone, the smallest and most fragile bone of the face, is situated at the front part of the medial wall of the orbit. It has two surfaces and four borders.

Mandible
Mandible
the bone forming the lower jaw; the largest and strongest bone of the face, presenting a body and a pair of rami, which articulate with the skull at the tempromandibular joints.

Maxillae
Maxillae
paired bones uniting to form the upper jawbone. The maxillae articulate with every bone of the face except the mandible, or lower jawbone. The maxilla (plural: maxillae), also known as the mustache bone, is a fusion of two bones along the palatal fissure that form the upper jaw. This is similar to the mandible (lower jaw), which is also a fusion of two halves at the mental symphysis. Sometimes (e.g. in bony fish), the maxilla is sometimes called “upper maxilla”, with the mandible being the “lower maxilla”. Conversely, in birds the upper jaw is often called “upper mandible”.

Nasal bone
Nasal bone
small oblong bones that meet at the middle and superior part of the face. Their fusion forms the superior part of the bridge of the nose.

Occipital bone
Occipital bone
a single trapezoid-shaped bone situated at the posterior and inferior part of the cranium. The occipital bone forms the back part of the skull and the base of the cranium. It joins with the parietal and temporal bones. In the center, underside (inferior) portion of the cranium, there is a large opening called the foramen magnum (fig. 3-5), through which nerve fibers from the brain pass and enter into the spinal cord. Figure 3-4.—Temporal bone. 3-2

Palatine bone
Palatine bone
a bone of extremely irregular form on each side of the skull that is situated in the posterior part of the nasal cavity between the maxilla and the pterygoid process of the sphenoid bone and that consists of a horizontal plate which joins the bone of the opposite side and forms the back part of the hard palate and a vertical plate which is extended into three processes and helps to form the floor of the orbit, the outer wall of the nasal cavity, and several adjoining parts called also palate bone palatine. One of two irregularly shaped bones (L-shaped) forming the posterior part of the hard palate, the lateral wall of the nasal fossa between the medial pterygoid plate and the maxilla, and the posterior part of the floor of the orbit. The posterior part of the hard palate, which separates the nasal cavity from the oral cavity, is formed by the horizontal plates.

Vomer
Vomer
a roughly triangular bone that forms the inferior and posterior of the nasal septum. The vomer is one of the unpaired facial bones of the skull. It is located in the midsagittal line, and articulates with the sphenoid, the ethmoid, the left and right palatine bones, and the left and right maxillary bones.

Parietal bones
Parietal bones
one of the two quadrilateral bones on either side of the cranium forming part of the superior and lateral surfaces of the skull, and joining each other in the midline at the sagittal suture. The parietal bones form the greater portion of the sides and roof of the cranial cavity.

Sphenoid bone
Sphenoid bone
The sphenoid bone (from Greek sphenoeides, “wedgelike”) is an unpaired bone situated at the base of the skull in front of the temporal bone and basilar part of the occipital bone. The sphenoid bone is one of the seven bones that articulate to form the orbit. Its shape somewhat resembles that of a butterfly or bat with its wings extended. A single, irregular, wedge-shaped bone at the base of the skull, which forms a part of the floor of the anterior, middle, and posterior cranial fossae. This bone is referred to as the keystone of the cranial floor because it articulates with all the other cranial bones.

Temporal bone
Temporal bone
one of the two irregular bones on either side of the skull forming part of the lateral surfaces and base of the skull, and containing the organs of hearing. The temporal bones form the inferior sides of the cranium and part of the cranial floor. The temporal bones are situated at the sides and base of the skull, and lateral to the temporal lobes of the cerebrum.

The temporal bone supports that part of the face known as the temple.

Zygomatic bone
Zygomatic bone
the triangular bones on either side of the face below the eyes, commonly referred to as the cheekbones, they form the prominences of the cheeks and part of the outer wall and floor of the orbits. It is situated at the upper and lateral part of the face and forms the prominence of the cheek, part of the lateral wall and floor of the orbit, and parts of the temporal and infratemporal fossae [Fig. 1]. It presents a malar and a temporal surface; four processes, the frontosphenoidal, orbital, maxillary, and temporal; and four borders.

Bone Morphology

Crest
a narrow prominent ridge.

Condyle
Condyle
a smooth rounded projection for articulation with another bone.

Epiphysis
Epiphysis
the end of a long bone that is originally separated from the main bone by a layer of cartilage but that later becomes united to the main bone through ossification [compare to suture and symphysis]. The epiphysis is the rounded end of a long bone, at its joint with adjacent bone(s). Between the epiphysis and diaphysis (the long midsection of the long bone) lies the metaphysis, including the epiphyseal plate (growth plate). At the joint, the epiphysis is covered with articular cartilage; below that covering is a zone similar to the epiphyseal plate, known as subchondral bone (see Wiktionary:subchondral).

The epiphysis is filled with red bone marrow, which produces erythrocytes (red blood cells).

Foramen
a true hole in the bone [e.g. foramen magnum, incisive foramen.
Foramen: A natural opening. Although a foramen is usually through bone, it can be an opening through other types of tissue, as with the foramen ovale in the heart. The plural of foramen is foramina.

Line
a narrow raised ridge.

Meatus
Meatus
a small tubular opening. In anatomy, a meatus is a natural body opening or canal (Latin, 4th declension pl. meatus, or meatuses – often incorrectly meati).

Examples include:

the external acoustic meatus, the opening of the ear canal
The internal auditory meatus, a canal in the temporal bone of the skull
the urinary meatus, which is the opening of the urethra, situated on the glans penis in males, and in the vulva in females the superior meatus, middle meatus and inferior meatus of the nose

Sulcus
Sulcus
a groove. In neuroanatomy, a sulcus (Latin: “furrow”, pl. sulci) is a depression or fissure in the surface of the brain. It surrounds the gyri, creating the characteristic appearance of the brain in humans and other large mammals.

Large furrows (sulci) that divide the brain into lobes are often called fissures. The large furrow that divides the two hemispheres—the interhemispheric fissure—is very rarely called a “sulcus”.

Suture
Suture
the line formed by the junction of two bones or an immovable joint between two bones, especially of the skull [compare to epiphysis and symphysis].

Symphysis
the line or junction formed by a cartilaginous articulation between two bones without an intervening synovial membrane, this articulation often fuses as in the two bones and the two halves of the mandibles [compare to suture and epiphysis].

Trochanter
Trochanter
Trochanter: One of the bony prominences toward the near end of the thigh bone (the femur). There are two trochanters:

•The greater trochanter: A powerful protrusion located at the proximal (near) and lateral (outside) part of the shaft of the femur;
•The lesser trochanter: A pyramidal prominence projecting from the proximal (near) and medial (inside) part of the shaft of the femur.
The greater trochanter is also called the major trochanter, the outer trochanter, and the lateral process of the femur. The lesser trochanter is alternatively called the minor trochanter, the inner trochanter and the medial process of the femur.

The trochanters are points at which hip and thigh muscles attach. The greater trochanter gives attachment to a number of muscles (including the gluteus medius and minimus, piriformis, obturator internus and externus, and gemelli muscles) while the lesser trochanter receives the insertion of several muscles (including the psoas major and iliacus [iliopsoas] muscles).
A large rounded projection for muscle attachment.

Histology Terms

adipose
adipose
A loose connective tissue made up of specialized cells called adipocytes, which store triglycerides. Adipose tissue is a major reserve of body energy and supports and protects various organs. In histology, adipose tissue or body fat or just fat is loose connective tissue composed of adipocytes. It is technically composed of roughly only 80% fat; fat in its solitary state exists in the liver and muscles. Adipose tissue is derived from lipoblasts. Its main role is to store energy in the form of fat, although it also cushions and insulates the body. Obesity or being overweight in humans and most animals does not depend on body weight but on the amount of body fat—to be specific, adipose tissue. Two types of adipose tissue exist: white adipose tissue (WAT) and brown adipose tissue (BAT). Adipose tissue also serves as an important endocrine organ[1] by producing hormones such as leptin, resistin, and the cytokine TNFα. The formation of adipose tissue appears to be controlled by the adipose gene. Adipose tissue was first identified by the Swiss naturalist Conrad Gessner in 1551.

anchoring junction
anchoring junction
a cellular junction which serves to anchor cells to one another or to extracellular material; seen in tissues subjected to friction and stretching (e.g., muscle tissue of heart). Bricks in a building must be stuck together and also tied somehow to the foundation. Similarly, cells within tissues and organs must be anchored to one another and attached to components of the extracellular matrix. Cells have developed several types of junctional complexes to serve these functions, and in each case, anchoring proteins extend through the plasma membrane to link cytoskeletal proteins in one cell to cytoskeletal proteins in neighboring cells as well as to proteins in the extracellular matrix.

Three types of anchoring junctions are observed, and differ from one another in the cytoskeletal protein anchor as well as the transmembrane linker protein that extends through the membrane:

apical surface
apical surface
The free or unattached surface of an epithelial cell. Epithelia are sheets of cells that provide the interface between masses of cells and a cavity or space (a lumen).
•The portion of the cell exposed to the lumen is called its apical surface.
•The rest of the cell (i.e., its sides and base) make up the basolateral surface.

Tight junctions seal adjacent epithelial cells in a narrow band just beneath their apical surface.

Tight junctions perform two vital functions:
•They prevent the passage of molecules and ions through the space between cells. So materials must actually enter the cells (by diffusion or active transport) in order to pass through the tissue. This pathway provides control over what substances are allowed through.
•They block the movement of integral membrane proteins (red and green ovals) between the apical and basolateral surfaces of the cell. Thus the special functions of each surface, for example
◦receptor-mediated endocytosis at the apical surface
◦exocytosis at the basolateral surface
can be preserved.

apocrine
apocrine
An exocrine gland which accumulates its secretory product at the apical surface of each cell. That part of each cell then separates from the remainder to from a secretion in the lumen of the gland. The cells then repair themselves (e.g., mammary glands). Apocrine is a term used to classify exocrine glands in the study of histology. Cells which are classified as apocrine bud their secretions off through the plasma membrane producing membrane bound vesicles in the lumen.

Apocrine glands are a type of human sweat gland that are present in areas such as the axillae (armpits), areola, in the perineum (genital areas), around the belly button and in the external auditory canal(as wax-secreting glands). Specialized types of apocrine glands present on the eyelids are called Moll’s glands. Apocrine sweat glands are inactive until they are stimulated by hormonal changes in puberty.

Apocrine glands secrete a milky, viscous odourless fluid which only develops a strong odour when it comes into contact with bacteria on the skin surface. Apocrine glands secrete this fluid by a method called decapitation secretion. That is, the apical portion of the secretory cell of the gland pinches off and enters the lumen of the gland. In contrast to this mechanism of secretion, Eccrine glands secrete by a method called merocrine secretion and sebaceous glands secrete by a method called holocrine secretion.

Apocrine sweat glands are mainly thought to function as olfactory pheremones, chemicals important in attracting a potential mate.

areolar
a widely distributed connective tissue made up of a number of different types of cells; combines with adipose tissue to form the subcutaneous layer of the body. Areolar tissue exhibits interlacing,[1] loosely organized fibers,[2] abundant blood vessels, and significant empty space. Its fiber run in random directions and are mostly collagenous, but elastic and reticular fibers are also present. Areolar tissue is highly variable in appearance. In many serous membranes, it appears as a loose arrangement of collagenous and elastic fibers, scattered cells of various types; abundant ground substance; numerous blood vessels. In the skin and mucous membranes, it is more compact and sometimes difficult to distinguish from dense irregular connective tissue. It is the most widely distributed connective tissue type in vertebrates.

It is sometimes equated with “loose connective tissue”.[3] In other cases, “loose connective tissue” is considered a parent category that includes mucous connective tissue, reticular connective tissue and adipose tissue.

basal lamina
the more superficial of two layers (along with the reticular lamina) making up the basement membrane. It is produced by the overlying epithelial tissue. 1 : the part of the gray matter of the embryonic neural tube from which the motor nerve roots arise
2 : a thin extracellular layer composed chiefly of collagen, proteoglycans, and glycoproteins (as laminin and fibronectin) that lies adjacent to the basal surface of epithelial cells or surrounds individual muscle, fat, and Schwann cells and that separates these cells from underlying or surrounding connective tissue or adjacent cells

basal surface
the surface of an epithelial cell which is attached to the underlying basement membrane.

basement membrane
the thin extracellular layer which attaches the epithelium to the underlying connective tissue. It is made up of a superficial basal lamina and an underlying reticular lamina.

-blast
a suffix which denotes a less differentiated precursor cell. It may or may not retain mitotic capability. Examples: myeloblast (white cell precursor), osteoblast (bone cell precursor).

cilia
cilia
hairlike projections of lining epithelial cells which help move substances through the lumen by a coordinated waving motion.

collagen
A protein which is the main component of connective tissue. Collagen is a group of naturally occurring proteins. In nature, it is found exclusively in animals.[1][clarification needed] It is the main protein of connective tissue. It is the most abundant protein in mammals,[2] making up about 25% to 35% of the whole-body protein content.

In muscle tissue it serves as a major component of endomysium. Collagen constitutes 1% to 2% of muscle tissue, and accounts for 6% of the weight of strong, tendinous muscles.[3] Gelatin, which is used in food and industry, is derived from collagen.

collagen fiber
The most common of three types of fiber embedded in the matrix between cells of connective tissue. These lie in parallel rows, and add great strength. Collagen fibers are an essential component of the body as it is a type of protein. Given below is some interesting information about these collagen fibers, so take a look.
As it is well known, collagen fibers are naturally occurring proteins found exclusively in animals and they are the main proteins in the connective tissues. Collagen fiber is the most commonly found protein in mammals and it makes up 25 to 35% of the whole body protein. Since 1930s scientists have been performing active research on the conformation of the collagen monomer which is sheet like or microfibrillar. This monomeric structure of the collagen fibers was described by Fraser, Miller and Wess with close observation. The collagen molecule is also called the ‘tropocollagen’ and is a aggregate of larger collagen fibrils. Given below is a detailed description about collagen fibers and their function.

Collagen Fiber Information

Tropocollagen, is approximately 300 nm in length and 1.5 nm in diameter. It is made up of polypeptide strands known as the alpha chains and each of them is a left handed helix in conformation. There are three such left handed helices which are twisted together to form a right handed triple helix or super helix which are bonded to each other with hydrogen bonds. This entire molecule which is collectively called the collagen fiber is made up of a regular arrangement of amino acids and hence it’s called a protein. Read on for information about the formation, types and function of these collagen fibers. More on collagen injections.

Collagen Fiber Types

Collagen is found in many parts of the body but the most common is the collagen fibers connective tissue. Throughout the body there are 29 types of collagen which have been identified till date. However more than 80% of the collagen in the body are of the types I, II, III and IV, each type found in different parts of the body.
•Collagen I is found in the skin, organs, bones and tendons.
•Collagen II is found in cartilages all over the body.
•Collagen III is a major component of reticular fibers and along type I.
•Collagen IV is found in the cell basement membrane.
The most commonly found collagen type is the first one and so we have discussed the formation of the collagen type I for you to get a better idea on how it is formed.

columnar
a type of epithelial tissue whose cells are shaped like columns. The tissue resembles a series of dominoes laid side by side.

connective tissue
The most common of four basic tissue types in the human body. Functions as support for epithelial tissues and as the binding (or “glue”) of various organs. The major types of connective tissue are: 1) loose connective tissue; 2) adipose tissue; 3) blood; 4) collagen, sometimes called fibrous or dense connective tissue; 5) cartilage; and 6) bone.

Loose connective tissue is a mass of widely scattered cells whose matrix is a loose weave of fibers. Many of the fibers are strong protein fibers called collagen. Loose connective tissue is found beneath the skin and between organs. It is a binding and packing material whose main purpose is to provide support to hold other tissues and organs in place.

Adipose tissue consists of adipose cells in loose connective tissue. Each adipose cell stores a large droplet of fat that swells when fat is stored and shrinks when fat is used to provide energy. Adipose tissue pads and insulates the animal body.

Blood is a loose connective tissue whose matrix is a liquid called plasma. Blood consists of red blood cells, erythrocytes, white blood cells, leukocytes, and thrombocytes or platelets, which are pieces of bone marrow cell. Plasma also contains water, salts, sugars, lipids, and amino acids. Blood is approximately 55 percent plasma and 45 percent formed elements. Blood transports substances from one part of the body to another and plays an important role in the immune system of the animal.

Collagen (from the Greek kolla, meaning “glue,” and genos, meaning “descent”) is a dense connective tissue, also known as fibrous connective tissue. It has a matrix of densely packed collagen fibers. There are two types of collagen: regular and irregular. The collagen fibers of regular dense connective tissue are lined up in parallel. Tendons, which bind muscle to bone, and ligaments, which join bones together, are examples of dense regularconnective tissue. The strong covering of various organs, such as kidneys and muscle, is dense irregular connective tissue.

Cartilage (from the Latin cartilago, meaning “gristle”) is a connective tissue with an abundant number of collagen fibers in a rubbery matrix. It is both strong and flexible. Cartilage provides support and cushioning. It is found between the discs of the vertebrae in the spine, surrounding the ends of joints such as knees, and in the nose and ears.

Bone is a rigid connective tissue that has a matrix of collagen fibers embedded in calcium salts. It is the hardest tissue in the body, although it is not brittle. Most of the skeletal system is comprised of bone, which provides support for muscle attachment and protects the internal organs.

cuboidal
cuboidal
A type of epithelial tissue whose cells are shaped like cubes or blocks.

cutaneous membrane
the membrane which covers the outer surface of the body (the skin).

-cyte
a suffix which denotes a differentiated or mature cell. It has usually lost its mitotic potential. Examples: osteocyte (mature bone cell), adipocyte (mature fat cell).

dermis
the connective tissue layer of skin.

desmosome
desmosome
A type of anchoring junction which forms a firm attachment with other cells in a manner resembling a “spot weld.” Desmosomes are molecular complexes of cell adhesion proteins and linking proteins that attach the cell surface adhesion proteins to intracellular keratin cytoskeletal filaments.

The cell adhesion proteins of the desmosome, desmoglein and desmocollin, are members of the cadherin family of cell adhesion molecules. They are transmembrane proteins that bridge the space between adjacent epithelial cells by way of homophilic binding of their extracellular domains to other desmosomal cadherins on the adjacent cell. Both have five extracellular domains, and have calcium-binding motifs.

The extracellular domain of the desmosome is called the Extracellular Core Domain (ECD) or the Desmoglea, and is bisected by an electron-dense midline where the desmoglein and desmocollin proteins bind to each other. These proteins can bind in a W, S, or λ manner.

On the cytoplasmic side of the plasma membrane, there are two dense structures called the Outer Dense Plaque (ODP) and the Inner Dense Plaque (IDP). These are spanned by the Desmoplakin protein.[1] The Outer Dense Plaque is where the cytoplasmic domains of the cadherins attach to desmoplakin via plakoglobin and plakophillin. The Inner Dense Plaque is where desmoplakin attaches to the intermediate filaments of the cell.

ectoderm
ectoderm
The outermost of the three primary embryonic germ layers, which gives rise to nervous tissue and the epidermis. The ectoderm is the outer layer of the embryo. It emerges first and forms from the outer layer of germ cells.

Generally speaking, the ectoderm differentiates to form the nervous system, tooth enamel and the epidermis (the outer part of integument).

In vertebrates, the ectoderm has three parts: external ectoderm (also known as surface ectoderm), the neural crest, and neural tube. The latter two are known as neuroectoderm.

elastic fiber
One of three types of fiber embedded in the matrix between cells of connective tissue. Smaller than collagen fibers, these allow tissues to stretch. Found in blood vessel walls and lung tissue.

endocrine
endocrine
A system of glands without ducts that deliver their secretions directly into the bloodstream.

endoderm
The innermost of the three primary embryonic germ layers, which gives rise to the GI tract, urinary bladder and urethra, and respiratory tract.

epidermis
the epithelial layer of skin.

epithelial membrane
a membrane made up of an epithelial tissue layer as well as an underlying connective tissue layer. Examples include cutaneous, mucous, and serous membranes.

epithelial tissue (epithelium)
the tissue which forms the superficial layer of skin and some organs. It also forms the inner lining of blood vessels, ducts, body cavities, and the interior of the respiratory, digestive, urinary, and reproductive systems.

exocrine
glands that deliver their secretions through ducts into body cavities or to the outside.

gap junction
a cellular junction which allows the two-way spread of action potentials from one cell to the next.

goblet cells
specialized epithelial cells which secrete mucous for lubrication.

ground substance
the chemical component of the matrix of connective tissue.

holocrine
an exocrine gland in which entire cells and their secretions accumulate as the gland’s secretory product. Discharged cells are replaced by new ones (e.g., sebaceous or oil glands).

hyaluronic acid
a complex molecule (glycosaminoglycan) which is one of several types of ground substance. It is a thick, lubricating substance which binds cells together and lubricates joints.

keratin
insoluble protein found in skin and epidermal appendages.

lumen
the space within a blood vessel or hollow organ.

Marfan’s syndrome
a hereditary disorder leading to a defect in elastic fibers.

matrix
the intercellular material, or “glue” of connective tissue, made up of protein fibers and ground substance.

membrane
a flexible sheet of tissue which may line cavities, or cover organs, joints, or the whole body.

merocrine
an exocrine gland which secretes its product from intact cells (e.g., salivary glands).

mesenchyme
the embryonic connective tissue from which all other connective tissue eventually arises.

mesoderm
the middle of the three primary embryonic germ layers, which gives rise to connective tissue and most muscle.

microvilli
microscopic projections of the plasma membrane of individual epithelial cells, to increase surface area in areas specialized for absorption or secretion (e.g., GI tract). Do not confuse these with villi (multicellular), or with cilia.

mucous membrane (mucosa)
the type of membrane which lines body cavities which are open to the exterior.

parenchyma
the functional tissue of an organ, as opposed to the supportive structures.

parietal layer
a layer of serous membrane which lines the inside of a body cavity.

pericardium
a loose serous membrane enclosing the heart.

peritoneum
the serous membrane which lines the abdominal cavity and covers the abdominal organs.

pleura
the serous membrane which lines the thoracic cavity and covers the lungs.

pseudostratified columnar
epithelial tissue made up of simple columnar cells whose nuclei are situated at different levels, giving the appearance of stratification (e.g. lining of trachea).

reticular fiber
one of three types of fiber embedded in the matrix between cells of connective tissue. Thinner than collagen fibers, this fiber forms branching networks, and helps form the stroma of many organs.

reticular lamina
the deeper of two layers (along with the basal lamina) making up the basement membrane. It is produced by the underlying connective tissue.

serous membrane (serosa)
the type of membrane which lines body cavities which do not open to the exterior. It also covers organs which lie within those cavities.

simple
an epithelial tissue consisting of a single row of cells.

simple columnar
an epithelial tissue consisting of a single row of columnar cells (e.g., inner lining of stomach).

simple cuboidal
an epithelial tissue consisting of a single row of cuboidal cells (e.g., thyroid follicles).

simple squamous
an epithelial tissue consisting of a single row of cells.

squamous
flat, tile-like cells.

stratified
consisting of many layers of cells.

stratified squamous
an epithelial tissue consisting of multiple layers of squamous cells (e.g., skin).

stroma
the supportive tissue of an organ, as opposed to the functioning tissue.

subcutaneous layer (superficial fascia)
a sheet made up of connective tissue and adipose tissue which lies just below the dermis of the skin, but above the deep fascia of muscle.

synovial membrane
a membrane which lines joints, which has only an areolar connective tissue layer.

tight junction
a cellular junction which forms a fluid-tight seal between cells (e.g., lining epithelial cells of large intestine).

transitional
a stratified epithelial tissue consisting of cells which vary in appearance between squamous and cuboidal. This depends on whether the tissue is stretched or relaxed (e.g., urinary bladder).

villus
a multicellular projection of lining epithelium in areas specialized for absorption or secretion.

visceral layer
a layer of serous membrane which lines the outside of a body organ.

Human Cells
Human Cells

Cell Structure & Function

Cell membranes | Cells, cytoplasm, & organelles | DNA & protein synthesis | Cell environment | Movement across membranes | Cellular metabolism

Evolution

Physiology
science that describes how organisms FUNCTION and survive in continually changing environments

Levels of Organization:

CHEMICAL LEVEL
CHEMICAL LEVEL
includes all chemical substances (atoms, ions, & molecules) necessary for life (e.g., genes and proteins or, shown below, a small portion – a heme group – of a hemoglobin molecule); together form the next higher level

CELLULAR LEVEL
CELLULAR LEVEL
cells are the basic structural and functional units of the human body & there are many different types of cells (e.g., muscle, nerve, blood, and so on)

TISSUE LEVEL
TISSUE LEVEL
a tissue is a group of cells that perform a specific function and the basic types of tissues in the human body include epithelial, muscle, nervous, and connective tissues

ORGAN LEVEL
an organ consists of 2 or more tissues that perform a particular function (e.g., heart, liver, stomach, and so on)

SYSTEM LEVEL
an association of organs that have a common function; there are 11 major systems in the human body, including digestive, nervous, endocrine, circulatory, respiratory, urinary, reproductive, muscular, lymphatic, skeletal, and integumentary.

Components of a cell

Two types of cells that make up all living things on earth: prokaryotic and eukaryotic. Prokaryotic cells, like bacteria, have no 'nucleus', while eukaryotic cells, like those of the human body, do. So, a human cell is enclosed by a cell, or plasma, membrane. Enclosed by that membrane is the cytoplasm (with associated organelles) plus a nucleus.
Two types of cells that make up all living things on earth: prokaryotic and eukaryotic. Prokaryotic cells, like bacteria, have no ‘nucleus’, while eukaryotic cells, like those of the human body, do. So, a human cell is enclosed by a cell, or plasma, membrane. Enclosed by that membrane is the cytoplasm (with associated organelles) plus a nucleus.
Two types of cells that make up all living things on earth: prokaryotic and eukaryotic. Prokaryotic cells, like bacteria, have no ‘nucleus’, while eukaryotic cells, like those of the human body, do. So, a human cell is enclosed by a cell, or plasma, membrane. Enclosed by that membrane is the cytoplasm (with associated organelles) plus a nucleus.

Structure of a typical bacterium

Structure of a typical eukaryotic cell
Structure of a typical eukaryotic cell

Cell, or Plasma, membrane
Cell, or Plasma, membrane
encloses every human cell

Structure
Structure
2 primary building blocks include protein (about 60% of the membrane) and lipid, or fat (about 40% of the membrane). The primary lipid is called phospholipid, and molecules of phospholipid form a ‘phospholipid bilayer’ (two layers of phospholipid molecules). This bilayer forms because the two ‘ends’ of phospholipid molecules have very different characteristics: one end is polar (or hydrophilic) and one (the hydrocarbon tails below) is non-polar (or hydrophobic):

supporting and retaining the cytoplasm
Functions include:
supporting and retaining the cytoplasm
being a selective barrier
The cell is separated from its environment and needs to get nutrients in and waste products out. Some molecules can cross the membrane without assistance, most cannot. Water, non-polar molecules and some small polar molecules can cross. Non-polar molecules penetrate by actually dissolving into the lipid bi-layer. Most polar compounds such as amino acids, organic acids and inorganic salts are not allowed entry, but instead must be specifically transported across the membrane by proteins.

transport
Many of the proteins in the membrane function to help carry out selective transport. These proteins typically span the whole membrane, making contact with the outside environment and the cytoplasm. They often require the expenditure of energy to help compounds move across the membrane

communication (via receptors)
communication (via receptors)
Membrane receptors generally function by binding the signal, or ligand and causing the production of a second signal (second messenger) that causes a cellular response. The diagram shows how the receptor for insulin functions.
A pair of insulin molecules bind to receptors, causing a change in the protein structure that exposes a kinase or phosphate transferring enzyme inside the cell.

The activated receptor kinase transfers a phosphate group to adjacent receptor molecules, leading to the activation of sub-cellular proteins and a cellular response to insulin.

The receptors must be integral membrane proteins that pass through the lipid bi layer. Through this system, an insulin molecule can cause a response inside the cell without passing through the lipid bi layer.

recognition
How do cells recognize each other?
Each of your cells has a set of ‘identity tags’ on its surface, which mark it out as part of your body and no one else’s. Some are only found on cells from the same tissue or organ. These identity tag molecules are called antigens. Your set of antigens is unique, unless you have an identical twin. Your immune system recognizes invading germs because they have unfamiliar antigens on their surfaces.

At the heart of the immune response is the ability to distinguish between self and non-self.
At the heart of the immune response is the ability to distinguish between self and non-self.
Every body cell carries distinctive molecules that distinguish it as “self.” Normally the body’s defenses do not attack tissues that carry a self marker; rather, immune cells coexist peaceably with other body cells in a state known as self-tolerance (Source: National Cancer Institute).

Tour of a cell

Cytoplasm consists of a gelatinous solution and contains microtubules (which serve as a cell's cytoskeleton) and organelles (literally 'little organs')
Cytoplasm consists of a gelatinous solution and contains microtubules (which serve as a cell’s cytoskeleton) and organelles (literally ‘little organs’)

The three fibers of the cytoskeleton-microtubules in blue, intermediate filaments in red, and actin in green-play countless roles in the cell.
The three fibers of the cytoskeleton-microtubules in blue, intermediate filaments in red, and actin in green-play countless roles in the cell.
The cyotoskeleton represents the cell’s skeleton. Like the bony skeletons that give us stability, the cytoskeleton gives our cells shape, strength, and the ability to move, but it does much more than that. The cytoskeleton is made up of three types of fibers that constantly shrink and grow to meet the needs of the cell: microtubules, microfilaments, and actin filaments. Each type of fiber looks, feels, and functions differently. Microtubules consists of a strong protein called tubulin and they are the ‘heavy lifters’ of the cytoskeleton. They do the tough physical labor of separating duplicate chromosomes when cells copy themselves and serve as sturdy railway tracks on which countless molecules and materials shuttle to and fro. They also hold the ER and Golgi neatly in stacks and form the main component of flagella and cilia.

Microfilaments are unusual because they vary greatly according to their location and function in the body. For example, some microfilaments form tough coverings, such as in nails, hair, and the outer layer of skin (not to mention animal claws and scales). Others are found in nerve cells, muscle cells, the heart, and internal organs. In each of these tissues, the filaments are made of different proteins.

Actin filament are made up of two chains of the protein actin twisted together. Although actin filaments are the most brittle of the cytoskeletal fibers, they are also the most versatile in terms of the shapes they can take. They can gather together into bundles, weblike networks, or even three-dimensional gels. They shorten or lengthen to allow cells to move and change shape. Together with a protein partner called myosin, actin filaments make possible the muscle contractions necessary for everything from your action on a sports field to the automatic beating of your heart.

In these cells, actin filaments appear light purple, microtubules yellow, and nuclei greenish blue.
In these cells, actin filaments appear light purple, microtubules yellow, and nuclei greenish blue.

Cells also contain a nucleus within which is found DNA (deoxyribonucleic acid) in the form of chromatin (or chromosomes during cell division) plus nucleoli (within which ribosomes are formed)
Cells also contain a nucleus within which is found DNA (deoxyribonucleic acid) in the form of chromatin (or chromosomes during cell division) plus nucleoli (within which ribosomes are formed)
The Nucleus and Nucleolus

The nucleus is the most obvious organelle in any eukaryotic cell. It is a membrane-bound organelle and is surrounded by a double membrane. It communicates with the surrounding cytosol via numerous nuclear pores.
Within the nucleus is the DNA responsible for providing the cell with its unique characteristics. The DNA is similar in every cell of the body, but depending on the specific cell type, some genes may be turned on or off – that’s why a liver cell is different from a muscle cell, and a muscle cell is different from a fat cell. When a cell is dividing, the DNA and surrounding protein condense into chromosomes (see photo) that are visible by microscopy.

The prominent structure in the nucleus is the nucleolus. The nucleolus produces ribosomes, which move out of the nucleus to positions on the rough endoplasmic reticulum where they are critical in protein synthesis.

Chromatin & chromosomes
Chromatin & chromosomes

Organelles include:

Endoplasmic reticulum
Endoplasmic reticulum
The endoplasmic reticulum (ER) is a special membrane structure found only in eukaryotic cells. Some ER has ribosomes on the surface (rough endoplasmic reticulum) –the cell’s protein-making machinery. Proteins that require special conditions or are destined to become part of the cell membrane are processed in the ER and then handed off to another organelle called the Golgi apparatus. The Golgi functions as a cellular post office. Proteins that arrive there are sorted, packaged and transported to various destinations in the cell. Scientists are studying many aspects of the ER and Golgi apparatus, including a built-in quality control mechanism cells use to ensure that proteins are properly made before leaving the ER

Golgi complex
Golgi complex
consists of a series of flattened sacs (or cisternae) functions include: synthesis (of substances likes phospholipids), packaging of materials for transport (in vesicles), and production of lysosomes The Golgi apparatus is a membrane-bound structure with a single membrane. It is actually a stack of membrane-bound vesicles that are important in packaging macromolecules for transport elsewhere in the cell. The stack of larger vesicles is surrounded by numerous smaller vesicles containing those packaged macromolecules. The enzymatic or hormonal contents of lysosomes, peroxisomes and secretory vesicles are packaged in membrane-bound vesicles at the periphery of the Golgi apparatus.

Lysosomes
Lysosomes
membrane-enclosed spheres that contain powerful digestive enzymes
functions include destruction of damaged cells (which is why they are sometimes called ‘suicide bags’) & digestion of phagocytosed materials (such as bacteria) . Lysosomes, Peroxisomes, Secretory Vesicles

A variety of small membrane-bound organelles differ primarily in their contents.
Lysosomes: Lysosomes (common in animal cells but rare in plant cells) contain hydrolytic enzymes necessary for intracellular digestion. In white blood cells that eat bacteria, lysosome contents are carefully released into the vacuole around the bacteria and serve to kill and digest those bacteria. Uncontrolled release of lysosome contents into the cytoplasm is also a component of necrotic cell death.

Peroxisomes: This organelle is responsible for protecting the cell from its own production of toxic hydrogen peroxide. As an example, white blood cells produce hydrogen peroxide to kill bacteria. The oxidative enzymes in peroxisomes break down the hydrogen peroxide into water and oxygen.

Secretory Vesicles: Cell secretions – e.g. hormones, neurotransmitters – are packaged in secretory vesicles at the Golgi apparatus. The secretory vesicles are then transported to the cell surface for release.

Mitochondria
Mitochondria
have a double-membrane: outer membrane & highly convoluted inner membrane. Mitochondria are the energy factories of the cells. The energy currency for the work that animals must do is the energy-rich molecule adenosine triphosphate (ATP). The ATP is produced in the mitochondria using energy stored in food. Just as the chloroplasts in plants act as sugar factories for the supply of ordered molecules to the plant, the mitochondria in animals and plants act to produce the ordered ATP molecules as the energy supply for the processes of life.

A typical animal cell will have on the order of 1000 to 2000 mitochondria. So the cell will have a lot of structures that are capable of producing a high amount of available energy. This ATP production by the mitochondria is done by the process of respiration, which in essence is the use of oxygen in a process which generates energy. This is a very efficient process for using food energy to make ATP. One of the benefits of “aerobic exercise” is that it improves your body’s ability to make ATP rapidly using the respiration process.

All living cells have mitochondria. Hair cells and outer skin cells are dead cells and no longer actively producing ATP, but all cells have the same structure. Some cells have more mitochondria than others. Your fat cells have many mitochondria because they store a lot of energy. Muscle cells have many mitochondria, which allows them to respond quickly to the need for doing work. Mitochondria occupy 15 to 20 percent of mammalian liver cells according to Karp.

Mitochondria are found exclusively in eukaryotic cells. These organelles are often called the “power plants” of the cell because their main job is to make energy (ATP). Mitochondria are highly unusual–they contain their own genetic material and protein-making machinery enwrapped in a double membrane. Many scientists believe mitochondria were once free-living bacteria that colonized complex cells sometime during evolution. (Source: NSF).

Ribosomes-
Ribosomes-
Ribosomes are the components of cells that make proteins from amino acids. primary function is to produce proteins. One of the central tenets of biology is that DNA makes RNA, which then makes protein. The DNA sequence in genes is copied into a messenger RNA (mRNA). Ribosomes then read the information in this RNA and use it to produce proteins. Ribosomes do this by binding to a messenger RNA and using it as a template for the correct sequence of amino acids in a particular protein. The amino acids are attached to transfer RNA (tRNA) molecules, which enter one part of the ribosome and bind to the messenger RNA sequence. The attached amino acids are then joined together by another part of the ribosome. The ribosome moves along the mRNA, “reading” its sequence and producing a chain of amino acids.

Ribosomes are made from complexes of RNAs and proteins. Ribosomes are divided into two subunits, one larger than the other. The smaller subunit binds to the mRNA, while the larger subunit binds to the tRNA and the amino acids. When a ribosome finishes reading a mRNA these two subunits split apart. Ribosomes have been classified as ribozymes, since the ribosomal RNA seems to be most important for the peptidyl transferase activity that links amino acids together.

Ribosomes from bacteria, archaea and eukaryotes (the three domains of life on Earth), have significantly different structure and RNA sequences. These differences in structure allow some antibiotics to kill bacteria by inhibiting their ribosomes, while leaving human ribosomes unaffected. The ribosomes in the mitochondria of eukaryotic cells resemble those in bacteria, reflecting the evolutionary origin of this organelle.[1] The word ribosome comes from ribonucleic acid and the Greek: soma (meaning body).

Centrioles
Centrioles
What are centrioles?

Centrioles are cylindrical structures that are composed of groupings of microtubules arranged in a 9 + 3 pattern. The pattern is so named because a ring of nine microtubule “triplets” are arranged at right angles to one another. Centrioles are found in animal cells and play a role in cell division. Centrioles replicate in interphase stage of mitosis and they help to organize the assembly of microtubules during cell division. Centrioles called “basal bodies” form cilia and flagella. located near the nucleus

Mitosis (cell division)
Mitosis (cell division)
Mitosis is the process by which a eukaryotic cell separates the chromosomes in its cell nucleus into two identical sets in two nuclei.[1] It is generally followed immediately by cytokinesis, which divides the nuclei, cytoplasm, organelles and cell membrane into two cells containing roughly equal shares of these cellular components. Mitosis and cytokinesis together define the mitotic (M) phase of the cell cycle – the division of the mother cell into two daughter cells, genetically identical to each other and to their parent cell. This accounts for approximately 10% of the cell cycle.

Mitosis occurs exclusively in eukaryotic cells, but occurs in different ways in different species. For example, animals undergo an “open” mitosis, where the nuclear envelope breaks down before the chromosomes separate, while fungi such as Aspergillus nidulans and Saccharomyces cerevisiae (yeast) undergo a “closed” mitosis, where chromosomes divide within an intact cell nucleus.[2] Prokaryotic cells, which lack a nucleus, divide by a process called binary fission.

The process of mitosis is complex and highly regulated. The sequence of events is divided into phases, corresponding to the completion of one set of activities and the start of the next. These stages are prophase, prometaphase, metaphase, anaphase and telophase. During the process of mitosis the pairs of chromosomes condense and attach to fibers that pull the sister chromatids to opposite sides of the cell. The cell then divides in cytokinesis, to produce two identical daughter cells.[3]

Because cytokinesis usually occurs in conjunction with mitosis, “mitosis” is often used interchangeably with “mitotic phase”. However, there are many cells where mitosis and cytokinesis occur separately, forming single cells with multiple nuclei. This occurs most notably among the fungi and slime moulds, but is found in various different groups. Even in animals, cytokinesis and mitosis may occur independently, for instance during certain stages of fruit fly embryonic development.[4] Errors in mitosis can either kill a cell through apoptosis or cause mutations that may lead to cancer.

Meiosis
Meiosis
In biology, meiosis (pronounced maɪˈoʊsɨs (help·info)) is a process of reductional division in which the number of chromosomes per cell is cut in half. In animals, meiosis always results in the formation of gametes, while in other organisms it can give rise to spores. As with mitosis, before meiosis begins, the DNA in the original cell is replicated during S-phase of the cell cycle. Two cell divisions separate the replicated chromosomes into four haploid gametes or spores.

Meiosis is essential for sexual reproduction and therefore occurs in all eukaryotes (including single-celled organisms) that reproduce sexually. A few eukaryotes, notably the Bdelloid rotifers, have lost the ability to carry out meiosis and have acquired the ability to reproduce by parthenogenesis. Meiosis does not occur in archaea or bacteria, which reproduce via asexual processes such as binary fission.

During meiosis, the genome of a diploid germ cell, which is composed of long segments of DNA packaged into chromosomes, undergoes DNA replication followed by two rounds of division, resulting in four haploid cells. Each of these cells contains one complete set of chromosomes, or half of the genetic content of the original cell. If meiosis produces gametes, these cells must fuse during fertilization to create a new diploid cell, or zygote before any new growth can occur. Thus, the division mechanism of meiosis is a reciprocal process to the joining of two genomes that occurs at fertilization. Because the chromosomes of each parent undergo homologous recombination during meiosis, each gamete, and thus each zygote, will have a unique genetic blueprint encoded in its DNA. Together, meiosis and fertilization constitute sexuality in the eukaryotes, and generate genetically distinct individuals in populations.

In all plants, and in many protists, meiosis results in the formation of haploid cells that can divide vegetatively without undergoing fertilization, referred to as spores. In these groups, gametes are produced by mitosis.

Meiosis uses many of the same biochemical mechanisms employed during mitosis to accomplish the redistribution of chromosomes. There are several features unique to meiosis, most importantly the pairing and recombination between homologous chromosomes.

Meiosis comes from the root -meio, meaning less.

Cilia and Flagella
Cilia and Flagella
These whiplike appendages extend from the surface of many types of eukaryotic cells.
•If there are many of them, they are called cilia;
•if only one, or a few, they are flagella. Flagella also tend to be longer than cilia but are otherwise similar in construction. Flagellum is relatively long and there’s typically just one Function
Cilia and flagella move liquid past the surface of the cell.
•For single cells, such as sperm, this enables them to swim.
•For cells anchored in a tissue, like the epithelial cells lining our air passages, this moves liquid over the surface of the cell (e.g., driving particle-laden mucus toward the throat).
Structure
Both cilia and flagella consist of:
•a cylindrical array of 9 filaments consisting of:
◦a complete microtubule extending into the tip of the cilium;
◦a partial microtubule that doesn’t extend as far into the tip.
◦cross-bridges of the motor protein dynein that extend from the complete microtubule of one filament to the partial microtubule of the adjacent filament.
•a pair of single microtubules running up through the center of the bundle, producing the “9+2” arrangement.
•The entire assembly is sheathed in a membrane that is an extension of the plasma membrane.
This electron micrograph (courtesy of Peter Satir) shows a cilium in cross section.

Each cilium (and flagellum) grows out from, and remains attached to, a basal body embedded in the cytoplasm. Basal bodies are identical to centrioles and are, in fact, produced by them. For example, one of the centrioles in developing sperm cells — after it has completed its role in the distribution of chromosomes during meiosis — becomes a basal body and produces the flagellum.

The Sliding-Filament Model of Bending
The bending of cilia (and flagella) has many parallels to the contraction of skeletal muscle fibers. Link to discussion of the sliding-filament model of skeletal muscle.
In the case of cilia and flagella, dynein powers the sliding of the microtubules against one another — first on one side, then on the other.

Villi
Villi
projections of cell membrane that serve to increase surface area of a cell (which is important, for example, for cells that line the intestine). In all humans, the villi together increase intestinal absorptive surface area approximately 30-fold and 60-fold, respectively, providing exceptionally efficient absorption of nutrients in the lumen. This increases the surface area so there are more places for food to be absorbed.

There are also enzymes on the surface for digestion. Villus capillaries collects amino acids and simple sugars taken up by the villi into the blood stream. Villus lacteals (Lymph capillary) collect absorbed chylomicrons, which are lipoproteins composed of triglycerides, cholesterol and amphipathic proteins, and are taken to the rest of the body through the Lymph fluid.

Zygote
Derived from the Greek meaning ‘yoked’, a zygote is the cell that results from fertilization. It is the union of a spermatozoon and an ovum — the mature germ cells, known also as the male and female gametes (from the Greek for husband and wife). Each of the two gametes is haploid, meaning that the nucleus has half the number of chromosomes of normal body cells. Their union results in the diploid zygote, with a full set of chromosomes, carrying the combination of genes that will determine all the bodily characteristics of the new individual. When, as a result of this union, matched genes (alleles) at particular sites on the newly paired chromosomes are different from each other, the zygote, and hence the resulting individual, is heterozygous with respect to those genes. It is homozygous if the pairs are identical. Since one of a dissimilar pair of genes can dominate the other, whereas identical pairs can act in unison, this is crucial to the suppression or emergence of the relevant inherited trait.

The zygote carries within its single cell continuing threads in the immemorial lifespan of the human race, as well as the mixed-and-matched microscopic material from which will stem the intricacies common to all human bodies, yet with the remarkable uniqueness of a particular person.

Small Intestine
Small Intestine
In vertebrates, the small intestine is the part of the gastrointestinal tract (gut) following the stomach and followed by the large intestine, and is where the vast majority of digestion and absorption of food takes place. In invertebrates such as worms, the terms “gastrointestinal tract” and “large intestine” are often used to describe the entire intestine. This article is primarily about the human gut, though the information about its processes are directly applicable to most mammals.[2] (A major exception to this are cows; for information about digestion in cows and other similar mammals, see ruminants.)

Size and divisions
The small intestine in an adult human measures on average about 5 meters (16 feet), with a normal range of 3 – 7 meters; it can measure around 50% longer at autopsy because of loss of smooth muscle tone after death. It is approximately 2.5-3 cm in diameter. Although the small intestine is much longer than the large intestine (typically around 3 times longer), it gets its name from its comparatively smaller diameter. Although as a simple tube the length and diameter of the small intestine would have a surface area of only about 0.5m2, the surface complexity of the inner lining of the small intestine increase its surface area by a factor of 500 to approximately 200m2, or roughly the size of a tennis court.

The small intestine is divided into three structural parts: ..

Duodenum 26 cm (9.8 in) in length
Jejunum 2.5 m (8.2 ft)
Ileum 3.5 m (11.5 ft)

DNA (Deoxyribonucleic acid)
DNA (Deoxyribonucleic acid)
controls cell function via transcription and translation (in other words, by controlling protein synthesis in a cell). DNA, or deoxyribonucleic acid, is the hereditary material in humans and almost all other organisms. Nearly every cell in a person’s body has the same DNA. Most DNA is located in the cell nucleus (where it is called nuclear DNA), but a small amount of DNA can also be found in the mitochondria (where it is called mitochondrial DNA or mtDNA). Tucked away inside the DNA of all of your genes are the instructions for how to construct a unique individual. Our genetic identity is “coded” in the sense that four building blocks, called nucleotides, string together to spell out a biochemical message—the manufacturing instructions for a protein. DNA’s four nucleotides, abbreviated A, T, G, and C, can only match up in specific pairs: A links to T and G links to C. An intermediate in this process, called mRNA (messenger ribonucleic acid), is made from the DNA template and serves as a link to molecular machines called ribosomes. Inside every cell, ribosomes read mRNA sequences and hook together protein building blocks called amino acids in the order specified by the code: Groups of three nucleotides in mRNA code for each of 20 amino acids. Connector molecules called tRNA (transfer RNA) aid in this process. Ultimately, the string of amino acids folds upon itself, adopting the unique shape that is the signature of that particular protein.

The information in DNA is stored as a code made up of four chemical bases: adenine (A), guanine (G), cytosine (C), and thymine (T). Human DNA consists of about 3 billion bases, and more than 99 percent of those bases are the same in all people. The order, or sequence, of these bases determines the information available for building and maintaining an organism, similar to the way in which letters of the alphabet appear in a certain order to form words and sentences.

DNA bases pair up with each other, A with T and C with G, to form units called base pairs. Each base is also attached to a sugar molecule and a phosphate molecule. Together, a base, sugar, and phosphate are called a nucleotide. Nucleotides are arranged in two long strands that form a spiral called a double helix. The structure of the double helix is somewhat like a ladder, with the base pairs forming the ladder’s rungs and the sugar and phosphate molecules forming the vertical sidepieces of the ladder.

An important property of DNA is that it can replicate, or make copies of itself. Each strand of DNA in the double helix can serve as a pattern for duplicating the sequence of bases. This is critical when cells divide because each new cell needs to have an exact copy of the DNA present in the old cell.

Transcription
Transcription
Transcription, or RNA synthesis, is the process of creating an equivalent RNA copy of a sequence of DNA[1]. Both RNA and DNA are nucleic acids, which use base pairs of nucleotides as a complementary language that can be converted back and forth from DNA to RNA in the presence of the correct enzymes. During transcription, a DNA sequence is read by RNA polymerase, which produces a complementary, antiparallel RNA strand. As opposed to DNA replication, transcription results in an RNA complement that includes uracil (U) in all instances where thymine (T) would have occurred in a DNA complement.

Transcription is the first step leading to gene expression. The stretch of DNA transcribed into an RNA molecule is called a transcription unit and encodes at least one gene. If the gene transcribed encodes for a protein, the result of transcription is messenger RNA (mRNA), which will then be used to create that protein via the process of translation. Alternatively, the transcribed gene may encode for either ribosomal RNA (rRNA) or transfer RNA (tRNA), other components of the protein-assembly process, or other ribozymes.

A DNA transcription unit encoding for a protein contains not only the sequence that will eventually be directly translated into the protein (the coding sequence) but also regulatory sequences that direct and regulate the synthesis of that protein. The regulatory sequence before (upstream from) the coding sequence is called the five prime untranslated region (5’UTR), and the sequence following (downstream from) the coding sequence is called the three prime untranslated region (3’UTR).

Transcription has some proofreading mechanisms, but they are fewer and less effective than the controls for copying DNA; therefore, transcription has a lower copying fidelity than DNA replication.[2]

As in DNA replication, DNA is read from 3′ → 5′ during transcription. Meanwhile, the complementary RNA is created from the 5′ → 3′ direction. Although DNA is arranged as two antiparallel strands in a double helix, only one of the two DNA strands, called the template strand, is used for transcription. This is because RNA is only single-stranded, as opposed to double-stranded DNA. The other DNA strand is called the coding strand, because its sequence is the same as the newly created RNA transcript (except for the substitution of uracil for thymine). The use of only the 3′ → 5′ strand eliminates the need for the Okazaki fragments seen in DNA replication.

Transcription is divided into 5 stages: pre-initiation, initiation, promoter clearance, elongation and termination.

Pre-initiation
In eukaryotes, RNA polymerase, and therefore the initiation of transcription, requires the presence of a core promoter sequence in the DNA. Promoters are regions of DNA which promote transcription and in eukaryotes, are found at -30, -75 and -90 base pairs upstream from the start site of transcription. Core promoters are sequences within the promoter which are essential for transcription initiation. RNA polymerase is able to bind to core promoters in the presence of various specific transcription factors.

Initiation
Initiation
In bacteria, a domain of prokaryotes, transcription begins with the binding of RNA polymerase to the promoter in DNA. RNA polymerase is a core enzyme consisting of five subunits: 2 α subunits, 1 β subunit, 1 β’ subunit, and 1 ω subunit. At the start of initiation, the core enzyme is associated with a sigma factor (number 70) that aids in finding the appropriate -35 and -10 base pairs downstream of promoter sequences.

Transcription initiation is more complex in eukaryotes. Eukaryotic RNA polymerase does not directly recognize the core promoter sequences. Instead, a collection of proteins called transcription factors mediate the binding of RNA polymerase and the initiation of transcription. Only after certain transcription factors are attached to the promoter does the RNA polymerase bind to it. The completed assembly of transcription factors and RNA polymerase bind to the promoter, forming a transcription initiation complex. Transcription in the archaea domain is similar to transcription in eukaryotes.

Promoter clearance
After the first bond is synthesized, the RNA polymerase must clear the promoter. During this time there is a tendency to release the RNA transcript and produce truncated transcripts. This is called abortive initiation and is common for both eukaryotes and prokaroytes[7]. Abortive initiation continues to occur until the σ factor rearranges, resulting in the transcription elongation complex (which gives a 35 bp moving footprint). The σ factor is released before 80 nucleotides of mRNA are synthesized[8]. Once the transcript reaches approximately 23 nucleotides, it no longer slips and elongation can occur. This, like most of the remainder of transcription, is an energy-dependent process, consuming adenosine triphosphate (ATP).

Promoter clearance coincides with phosphorylation of serine 5 on the carboxy terminal domain of RNA Pol in eukaryotes, which is phosphorylated by TFIIH.

Elongation
Elongation
One strand of DNA, the template strand (or noncoding strand), is used as a template for RNA synthesis. As transcription proceeds, RNA polymerase traverses the template strand and uses base pairing complementarity with the DNA template to create an RNA copy. Although RNA polymerase traverses the template strand from 3′ → 5′, the coding (non-template) strand and newly-formed RNA can also be used as reference points, so transcription can be described as occurring 5′ → 3′. This produces an RNA molecule from 5′ → 3′, an exact copy of the coding strand (except that thymines are replaced with uracils, and the nucleotides are composed of a ribose (5-carbon) sugar where DNA has deoxyribose (one less oxygen atom) in its sugar-phosphate backbone).

Unlike DNA replication, mRNA transcription can involve multiple RNA polymerases on a single DNA template and multiple rounds of transcription (amplification of particular mRNA), so many mRNA molecules can be rapidly produced from a single copy of a gene.

Elongation also involves a proofreading mechanism that can replace incorrectly incorporated bases. In eukaryotes, this may correspond with short pauses during transcription that allow appropriate RNA editing factors to bind. These pauses may be intrinsic to the RNA polymerase or due to chromatin structure.

Termination
Bacteria use two different strategies for transcription termination. In Rho-independent transcription termination, RNA transcription stops when the newly synthesized RNA molecule forms a G-C rich hairpin loop followed by a run of Us, which makes it detach from the DNA template. In the “Rho-dependent” type of termination, a protein factor called “Rho” destabilizes the interaction between the template and the mRNA, thus releasing the newly synthesized mRNA from the elongation complex.

Transcription termination in eukaryotes is less understood but involves cleavage of the new transcript followed by template-independent addition of As at its new 3′ end, in a process called polyadenylation.

Translation
Translation is the first stage of protein biosynthesis (part of the overall process of gene expression). In translation, messenger RNA (mRNA) produced in transcription is decoded to produce a specific amino acid chain, or polypeptide, that will later fold into an active protein. Translation occurs in the cell’s cytoplasm, where the large and small subunits of the ribosome are located, and bind to the mRNA. The ribosome facilitates decoding by inducing the binding of tRNAs with complementary anticodon sequences to that of the mRNA. The tRNAs carry specific amino acids that are chained together into a polypeptide as the mRNA passes through and is “read” by the ribosome in a fashion reminiscent to that of a stock ticker and ticker tape.

A ribosome translating a protein that is secreted into the endoplasmic reticulum. tRNAs are colored dark blue.In many instances, the entire ribosome/mRNA complex will bind to the outer membrane of the rough endoplasmic reticulum and release the nascent protein polypeptide inside for later vesicle transport and secretion outside of the cell. Many types of transcribed RNA, such as transfer RNA, ribosomal RNA, and small nuclear RNA, do not undergo translation into proteins.

Translation proceeds in four phases: activation, initiation, elongation and termination (all describing the growth of the amino acid chain, or polypeptide that is the product of translation). Amino acids are brought to ribosomes and assembled into proteins.

In activation, the correct amino acid is covalently bonded to the correct transfer RNA (tRNA). The amino acid is joined by its carboxyl group to the 3′ OH of the tRNA by a peptide bond. When the tRNA has an amino acid linked to it, it is termed “charged”. Initiation involves the small subunit of the ribosome binding to 5′ end of mRNA with the help of initiation factors (IF). Termination of the polypeptide happens when the A site of the ribosome faces a stop codon (UAA, UAG, or UGA). No tRNA can recognize or bind to this codon. Instead, the stop codon induces the binding of a release factor protein that prompts the disassembly of the entire ribosome/mRNA complex.

A number of antibiotics act by inhibiting translation; these include anisomycin, cycloheximide, chloramphenicol, tetracycline, streptomycin, erythromycin, and puromycin, among others. Prokaryotic ribosomes have a different structure from that of eukaryotic ribosomes, and thus antibiotics can specifically target bacterial infections without any detriment to a eukaryotic host’s cells.

Membranes, Glands, and Cartilage
Mucous, serous, synovial, and cutaneous are the principal kinds of membranes and are composed mainly of epithelial tissue. Types of glands include sudoriferous, sebaceous, and ceruminous. Cartilage is replaced by bone in embryonic development and is found mainly in the joints, the thorax, and various rigid tubes.

Epithelial Tissues
Epithelial Tissues
Epithelial tissue covers the whole surface of the body. It is made up of cells closely packed and ranged in one or more layers. This tissue is specialised to form the covering or lining of all internal and external body surfaces. Epithelial tissue that occurs on surfaces on the interior of the body is known as endothelium. Epithelial cells are packed tightly together, with almost no intercellular spaces and only a small amount of intercellular substance. Epithelial tissue, regardless of the type, is usually separated from the underlying tissue by a thin sheet of connective tissue; basement membrane. The basement membrane provides structural support for the epithelium and also binds it to neighbouring structures.

Types of Epithelial Tissue
Epithelial tissue can be divided into two groups depending on the number of layers of which it is composes. Epithelial tissue which is only one cell thick is known as simple epithelium. If it is two or more cells thick such as the skin, it is known as stratified epithelium.

Skin
Skin
The skin is the outer covering of the body. In humans, it is the largest organ of the integumentary system made up of multiple layers of ectodermal tissue, and guards the underlying muscles, bones, ligaments and internal organs.[1] Human skin is not unlike that of most other mammals except that it is not protected by a pelt and appears hairless though in fact nearly all human skin is covered with hair follicles. There are two general types of skin, hairy and glabrous skin.[2] The adjective cutaneous literally means “of the skin” (from Latin cutis, skin).

Because it interfaces with the environment, skin plays a key role in protecting (the body) against pathogens[3] and excessive water loss.[4] Its other functions are insulation, temperature regulation, sensation, synthesis of vitamin D, and the protection of vitamin B folates. Severely damaged skin will try to heal by forming scar tissue. This is often discolored and depigmented.

In humans, skin pigmentation varies among populations, and skin type can range from dry to oily. Such skin variety provides a rich and diverse habit for bacteria which number roughly a 1000 species from 19 phyla

Skeletal System
Skeletal System
The human skeleton consists of both fused and individual bones supported and supplemented by ligaments, tendons, muscles and cartilage. It serves as a scaffold which supports organs, anchors muscles, and protects organs such as the brain, lungs and heart. The biggest bone in the body is the femur in the upper leg, and the smallest is the stapes bone in the middle ear. In an adult, the skeleton comprises around 14% of the total body weight,[1] and half of this weight is water.

Fused bones include those of the pelvis and the cranium. Not all bones are interconnected directly: There are three bones in each middle ear called the ossicles that articulate only with each other. The hyoid bone, which is located in the neck and serves as the point of attachment for the tongue, does not articulate with any other bones in the body, being supported by muscles and ligaments.

Joints
A joint, or articulation, is the place where two bones come together. There are three types of joints classified by the amount of movement they allow: immovable, slightly movable, and freely movable.

Immovable joints are synarthroses. In this type of joint, the bones are in very close contact and are separated only by a thin layer of fibrous connective tissue. An example of a synarthrosis is the suture in the skull between skull bones.

Slightly movable joints are called amphiarthroses. This type of joint is characterized by bones that are connected by hyaline cartilage (fibro cartilage). The ribs that connect to the sternum are an example of an amphiarthrosis joint.

Most of the joints in the adult human body are freely movable joints. This type of joint is called a diarthrosis joint. There are six types of diarthroses joints. These are:

Ball-and-Socket: The ball-shaped end of one bone fits into a cup shaped socket on the other bone allowing the widest range of motion including rotation. Examples include the shoulder and hip.

Condyloid: Oval shaped condyle fits into elliptical cavity of another allowing angular motion but not rotation. This occurs between the metacarpals (bones in the palm of the hand) and phalanges (fingers) and between the metatarsals (foot bones excluding heel) and phalanges (toes).

Saddle: This type of joint occurs when the touching surfaces of two bones have both concave and convex regions with the shapes of the two bones complementing one other and allowing a wide range of movement. The only saddle joint in the body is in the thumb.

Pivot: Rounded or conical surfaces of one bone fit into a ring of one or tendon allowing rotation. An example is the joint between the axis and atlas in the neck.

Hinge: A convex projection on one bone fits into a concave depression in another permitting only flexion and extension as in the elbow joints.

Gliding: Flat or slightly flat surfaces move against each other allowing sliding or twisting without any circular movement. This happens in the carpals in the wrist and the tarsals in the ankle.

Axial Skeleton: Skull, Sternum, Ribs, Vertebral Column
Axial Skeleton: Skull, Sternum, Ribs, Vertebral Column
The skull is the bony framework of the head. It is comprised of the eight cranial and fourteen facial bones.

Cranial Bones

The cranial bones makeup the protective frame of bone around the brain.
The cranial bones are:

The frontal forms part of the cranial cavity as well as the forehead, the brow ridges and the nasal cavity.
The left and right parietal forms much of the superior and lateral portions of the cranium.
The left and right temporal form the lateral walls of the cranium as well as housing the external ear.
The occipital forms the posterior and inferior portions of the cranium. Many neck muscles attach here as this is the point of articulation with the neck.
The sphenoid forms part of the eye orbit and helps to form the floor of the cranium.
The ethmoid forms the medial portions of the orbits and the roof of the nasal cavity.
The joints between bones of the skull are immovable and called sutures. The parietal bones are joined by the sagittal suture. Where the parietal bones meet the frontal is referred to as the coronal suture. The parietals and the occipital meet at the lambdoidal suture. The suture between the parietals and the temporal bone is referred to as the squamous suture. These sites are the common location of fontanelles or “soft spots” on a baby’s head.

Facial Bones

The facial bones makeup the upper and lower jaw and other facial structures.
The facial bones are:

The mandible is the lower jawbone. It articulates with the temporal bones at the temporomandibular joints. This forms the only freely moveable joint in the head. It provides the chewing motion.
The left and right maxilla are the upper jaw bones. They form part of the nose, orbits, and roof of the mouth.
The left and right palatine form a portion of the nasal cavity and the posterior portion of the roof of the mouth.
The left and right zygomatic are the cheek bones. They form portions of the orbits as well.
The left and right nasal form the superior portion of the bridge of the nose.
The left and right lacrimal help to form the orbits.
The vomer forms part of the nasal septum (the divider between the nostrils).
The left and right inferior turbinate forms the lateral walls of the nose and increase the surface area of the nasal cavity.

Sternum
Sternum
The sternum is a flat, dagger shaped bone located in the middle of the chest. Along with the ribs, the sternum forms the rib cage that protects the heart, lungs, and major blood vessels from damage.

The sternum is composed of three parts:

The manubrim, also called the “handle”, is located at the top of the sternum and moves slightly. It is connected to the first two ribs.

The body, also called the “blade” or the “gladiolus”, is located in the middle of the sternum and connects the third to seventh ribs directly and the eighth through tenth ribs indirectly.

The xiphoid process, also called the “tip”, is located on the bottom of the sternum. It is often cartilaginous (cartilage), but does become bony in later years.

These three segments of bone are usually fused in adults.

The sternum serves an important function in the body. The ribs are connected to it by the costal cartilage. Without the sternum, there would be a hole in the bone structure in the middle of your chest, right above your heart and lungs. The sternum protects this vital area and completes the circle of the rib cage.

Ribs The clavicle in the shoulder is the most commonly broken bone in the body because it transmits forces from the arm to the trunk.
Ribs The clavicle in the shoulder is the most commonly broken bone in the body because it transmits forces from the arm to the trunk.
The ribs are thin, flat, curved bones that form a protective cage around the organs in the upper body. They are comprised 24 bones arranged in 12 pairs.

These bones are divided into three categories:

The first seven bones are called the true ribs. These bones are connected to the spine (the backbone) in back. In the front, the true ribs are connected directly to the breastbone or sternum by a strips of cartilage called the costal cartilage.

The next three pairs of bones are called false ribs. These bones are slightly shorter than the true ribs and are connected to the spine in back. However, instead of being attached directly to the sternum in front, the false ribs are attached to the lowest true rib.

The last two sets of rib bones are called floating ribs. Floating ribs are smaller than both the true ribs and the false ribs. They are attached to the spine at the back, but are not connected to anything in the front.

The ribs form a kind of cage the encloses the upper body. They give the chest its familiar shape.

The ribs serve several important purposes. They protect the heart and lungs from injuries and shocks that might damage them. Ribs also protect parts of the stomach, spleen, and kidneys. The ribs help you to breathe. As you inhale, the muscles in between the ribs lift the rib cage up, allowing the lungs to expand. When you exhale, the rib cage moves down again, squeezing the air out of your lungs.

Vertebral Column
Vertebral Column
The vertebral column (also called the backbone, spine, or spinal column) consists of a series of 33 irregularly shaped bones, called vertebrae. These 33 bones are divided into five categories depending on where they are located in the backbone.

The first seven vertebrae are called the cervical vertebrae. Located at the top of the spinal column, these bones form a flexible framework for the neck and support the head. The first cervical vertebrae is called the atlas and the second is called the axis. The atlas’ shape allows the head to nod “yes” and the axis’ shape allows the head to shake “no”.

The next twelve vertebrae are called the thoracic vertebrae. These bones move with the ribs to form the rear anchor of the rib cage. Thoracic vertebrae are larger than cervical vertebrae and increase in size from top to bottom.

After the thoracic vertebrae, come the lumbar vertebrae. These five bones are the largest vertebrae in the spinal column. These vertebrae support most of the body’s weight and are attached to many of the back muscles.

The sacrum is a triangular bone located just below the lumbar vertebrae. It consists of four or five sacral vertebrae in a child, which become fused into a single bone after age 26. The sacrum forms the back wall of the pelvic girdle and moves with it.

The bottom of the spinal column is called the coccyx or tailbone. It consists of 3-5 bones that are fused together in an adult. Many muscles connect to the coccyx.

These bones compose the vertebral column, resulting in a total of 26 movable parts in an adult. In between the vertebrae are intervertebral discs made of fibrous cartilage that act as shock absorbers and allow the back to move. As a person ages, these discs compress and shrink, resulting in a distinct loss of height (generally between 0.5 and 2.0cm) between the ages of 50 and 55.

When looked at from the side, the spine forms four curves. These curves are called the cervical, thoracic, lumbar, and pelvic curves. The cervical curve is located at the top of the spine and is composed of cervical vertebrae. Next come the thoracic and lumbar curves composed of thoracic and lumbar vertebrae respectively. The final curve called the pelvic or sacral curve is formed by the sacrum and coccyx. These curves allow human beings to stand upright and help to maintain the balance of the upper body. The cervical and lumbar curves are not present in an infant. The cervical curves forms around the age of 3 months when an infant begins to hold its head up and the lumbar curve develops when a child begins to walk.

In addition to allowing humans to stand upright and maintain their balance, the vertebral column serves several other important functions. It helps to support the head and arms, while permitting freedom of movement. It also provides attachment for many muscles, the ribs, and some of the organs and protects the spinal cord, which controls most bodily functions.

Appendicular Skeleton: The Upper Extremities
The Lower Extremities
The Shoulder Girdle
The Pelvic Girdle–(the sacrum and coccyx are considered part of the vertebral column)
The appendicular skeleton is composed of bones that anchor the appendages to the axial skeleton.

The Upper Extremities
The Upper Extremities
The upper extremity consists of three parts: the arm, the forearm, and the hand.

The Arm
The arm, or brachium, is technically only the region between the shoulder and elbow. It consists of a single long bone called the humerus. The humerus is the longest bone in the upper extremity. The top, or head, is large, smooth, and rounded and fits into the scapula in the shoulder. On the bottom of the humerus, are two depressions where the humerus connects to the ulna and radius of the forearm. The radius is connected on the side away from the body (lateral side) and the ulna is connected on the side towards the body (medial side) when standing in the anatomical position. Together, the humerus and the ulna make up the elbow. The bottom of the humerus protects the ulnar nerve and is commonly known as the “funny bone” because striking the elbow on a hard surface stimulates the ulnar nerve and produces a tingling sensation.

The Forearm
The forearm is the region between the elbow and the wrist. It is formed by the radius on the lateral side and the ulna on the medial side when the forearm is viewed in the anatomical position. The ulna is longer than the radius and connected more firmly to the humerus. The radius, however, contributes more to the movement of the wrist and hand than the ulna. When the hand is turned over so that the palm is facing downwards, the radius crosses over the ulna. The top of each bone connects to the humerus of the arm and the bottom of each connects to the bones of the hand.

The Hand
The hand consists of three parts (the wrist, palm, and five fingers) and 27 bones.

The wrist, or carpus, consists of 8 small bones called the carpal bones that are tightly bound by ligaments. These bone are arranged in two rows of four bones each. The top row (the row closest to the forearm) from the lateral (thumb) side to the medial side contains the scaphoid, lunate, triquetral, and pisiform bones. The second row from lateral to medial contains the trapezium, trapezoid, capitate, and hamate. The scaphoid and lunate connect to the bottom of the radius.

The palm or metacarpus consists of five metacarpal bones, one aligned with each of the fingers. The metacarpal bones are not named but are numbered I to V starting with the thumb. The bases of the metacarpal bones are connected to the wrist bones and the heads are connected to the bones of the fingers. The heads of the metacarpals form the knuckles of a clenched fist.

The fingers are made up of 14 bones called phalanges. A single finger bone is called a phalanx. The phalanges are arranged in three rows. The first row (the closest to the metacarpals) is called the proximal row, the second row is the middle row, and the farthest row is called the distal row. Each finger has a proximal phalanx, a middle phalanx, and a distal phalanx, except the thumb (also called the pollex) which does not have a middle phalanx. The digits are also numbered I to V starting from the thumb.

The Lower Extremities
The lower extremity is composed of the bones of the thigh, leg, foot, and the patella (commonly known as the kneecap).

The Thigh

The thigh is the region between the hip and the knee and is composed of a single bone called the femur or thighbone. The femur is the longest, largest, and strongest bone in the body.

The Leg

The leg is technically only the region from the knee to the ankle. It is formed by the fibula on side away from the body (lateral side) and the tibia, also called the shin bone, on the side nearest the body (medial side). The tibia connects to the femur to form the knee joint and with the talus, a foot bone, to allow the ankle to flex and extend. The tibia is larger than the fibula because it bears most of the weight, while the fibula serves as an area for muscle attachment.

The Foot

The foot, or pes, contains the 26 bones of the ankle, instep, and the five toes. The ankle, or tarsus, is composed of the 7 tarsal bones which correspond to the carpals in the wrist. The largest tarsal bone is called the calcaneus or heel bone. The talus rests on top of the calcaneus and is connected to the tibia. Directly in front of the talus is the navicular bone. The remaining bones from medial to lateral are the medial, intermediate, the lateral cuneiform bones, and the cuboid bone.

The metatarsal and phalanges bones of the foot are similar in number and position to the metacarpal and phalanges bones of the hand. The five metatarsal bones are numbered I to V starting on the medial side with the big toe. The first metatarsal bone is larger than the others because it plays a major role in supporting the body’s weight. The 14 phalanges of the foot, as with the hand, are arranged in a proximal row, a middle row, and a distal row, with the big toe, or hallux, having only a proximal and distal phalanx.

The foot’s two arches are formed by the structure and arrangement of the bones and are maintained by tendons and ligaments. The arches give when weight is placed on the foot and spring back when the weight is lifted off of the foot. The arches may fall due to a weakening of the ligaments and tendons in the foot.

The Patella

The patella or kneecap is a large, triangular sesamoid bone between the femur and the tibia. It is formed in response to the strain in the tendon that forms the knee. The patella protects the knee joint and strengthens the tendon that forms the knee.

The bones of the lower extremities are the heaviest, largest, and strongest bones in the body because they must bear the entire weight of the body when a person is standing in the upright position.

The Shoulder Girdle
The Shoulder Girdle
The Shoulder Girdle, also called the Pectoral Girdle, is composed of four bones: two clavicles and two scapulae .

The clavicle, commonly called the collarbone, is a slender S-shaped bone that connects the upper arm to the trunk of the body and holds the shoulder joint away from the body to allow for greater freedom of movement. One end of the clavicle is connected to the sternum and one end is connected to the scapula.

Thescapula is a large, triangular, flat bone on the back side of the rib cage commonly called the shoulder blade. It overlays the second through seventh rib and serves as an attachment for several muscles. It has a shallow depression called the glenoid cavity that the head of the humerus (upper arm bone) fits into.

Usually, a “girdle” refers to something that encircles or is a complete ring. However, the shoulder girdle is an incomplete ring. In the front, the clavicles are separated by the sternum. In the back, there is a gap between the two scapulae.

The primary function of the pectoral girdle is to provide an attachment point for the numerous muscles that allow the shoulder and elbow joints to move. It also provides the connection between the upper extremities (the arms) and the axial skeleton.

The Pelvic Girdle
The Pelvic Girdle
The Pelvic Girdle, also called the hip girdle, is composed to two coxal (hip) bones. The coxal bones are also called the ossa coxae or innominate bones. During childhood, each coxal bone consists of three separate parts: the ilium (denoted in purple above), the ischium (denoted in red above), and the pubis (denoted in blue above). In an adult, these three bones are firmly fused into a single bone. In the picture above, the coxal bone on the left side has been divided into its component pieces while the right side has been preserved.

In the back, these two bones meet on either side of the sacrum. In the front, they are connected by a muscle called the pubic symphysis (denoted in green above).

The pelvic girdle serves several important functions in the body. It supports the weight of the body from the vertebral column. It also protects and supports the lower organs, including the urinary bladder, the reproductive organs, and the developing fetus in a pregnant woman.

The pelvic girdle differs between men and woman. In a man, the pelvis is more massive and the iliac crests are closer together. In a woman, the pelvis is more delicate and the iliac crests are farther apart. These differences reflect the woman’s role in pregnancy and delivery of children. When a child is born, it must pass through its mother’s pelvis. If the opening is too small, a cesarean section may be necessary.

**The picture above is of a female pelvis. All divisions are approximations only.**

Bone Cells
There are five main types of bone cells in bone tissue. Osteogenic cells respond to traumas, such as fractures, by giving rise to bone-forming cells and bone-destroying cells. Osteoblasts (bone-forming cells) synthesize and secrete unmineralized ground substance and are found in areas of high metabolism within the bone. Osteocytes are mature bone cells made from osteoblasts that have made bone tissue around themselves. These cells maintain healthy bone tissue by secreting enzymes and controlling the bone mineral content; they also control the calcium release from the bone tissue to the blood. Osteoclasts are large cells that break down bone tissue. They are very important to bone growth, healing, and remodeling. The last type of cells are bone-lining cells. These are made from osteoblasts along the surface of most bones in an adult. Bone-lining cells are thought to regulate the movement of calcium and phosphate into and out of the bone.

Human Anatomy Terms
Human Anatomy Terms
The following terms are those which are used to identify the location of parts of the human body in medicine and academic study. These terms are often used to describe a specific portion of a structure or to compare the locations of two different structures. “The hand is distal to the forearm” or “the medial portion of the frontal bone contains the frontal sinus” are examples of this. We have organized this list of terms by keeping similar pairs or groups of terms together instead of by alphabetical order so that you will find them easier to learn and remember.

Superior – toward the head

Inferior – away from the head

Anterior – the front of the body or body part

Posterior – the back of the body or body part

Medial – toward the midline that divides left and right

Lateral – to the side away from the midline

Proximal – closer to the torso

Distal – farther away from the torso

Anatomical position – standing erect, facing the observer, arms are at the sides with palms facing forward.

Muscular System
The human muscular system comprises of more than 600 muscles. Read more about them here.
The human muscular system is made up of over 600 muscles, which act in groups. Muscles, in turn, are made up of fibers and cells. Muscles are what enable you to do just about everything – from walking to lifting heavy objects to helping to pump blood throughout the body. Muscles are distinguished as either involuntary or voluntary. Involuntary muscles function within the body automatically, without you being able to control them. Voluntary muscles are the ones that are under your control.

All muscles are made up of the same type of material – a kind of an elastic tissue, akin to what rubber bands are made of. Each muscle is made up of thousands of tiny fibers. There are three kinds of muscles in the human muscular system: the skeletal muscle; the cardiac muscle; and the smooth muscle. Plus, the facial muscles and the tongue are a unique kind by themselves.

The Skeletal Muscles: These are the voluntary type of muscles in the human muscular system. This means that they can be controlled by you. For example, you cannot pick up that mug of coffee with your hand unless you want your hand to do so. They are referred sometimes as striated muscles, because the dark and light fibrous material make them seem striped. These are also known as the musculoskeletal system, or the combination of the muscles and the bones that make up the skeleton.

Generally, skeletal muscles are attached to the ends of bones, stretching all across the joint and then attached once more to another bone. Tendons, which are cords or bands of inelastic tissue, are what attach the muscles to the bones. Skeletal muscles are of different shapes and sizes, which enable them to perform a variety of tasks. The gluteus maximus, or the muscle that occurs in the buttocks, is the largest skeletal muscle in the human muscular system. Some of the other major skeletal muscles are the deltoid muscle in the shoulders, the biceps and triceps in the arm, the pectoralis in the chest, the rectus abdominus in the abdomen, the quadriceps and the hamstring muscles in the legs.

The Cardiac Muscle: The heart is made up of the cardiac muscle, which is also referred to as the myocardium. These muscles are thick and contract in order to pump out the blood and then relax in order to allow more blood in. The cardiac muscle is an involuntary muscle, or the type that works without your volition. Special type of cells in the cardiac muscle, called the pacemaker, help in controlling the heartbeat.

The Smooth Muscles: These are the involuntary muscles of the human muscular system, and they generally occur in layers or sheets, with one muscle layer behind another. These muscles are not under your control. The brain and the body control these muscles in performing their functions without any conscious volition from your part.

Some of the examples of smooth muscles are the stomach and the digestive system, which contract and relax in order to pass food through the alimentary canal of the body. The bladder is another example of smooth muscle, and so is the uterus in women. Smooth muscles also occur in the eyes, which help to keep the eyes focused. According to scientists, the eyes can move over 100,000 times in a day, making them the busiest muscles in the human muscular system.

The Facial Muscles: There are more than 30 muscles in the face. Not all of the facial muscles are attached to bones, as is the case in the other parts of the body. Many of the facial muscles are attached to the underside of facial skin. The contractions of these muscles are what give the face its various expressions, such as frowning, laughter, surprise, sadness and so on.

The Tongue: And another unique muscle is the tongue, which is free at one end and only attached on the other end. The tongue actually comprises of a group of muscles, which work in unison, enabling you to chew and swallow food, and talk.

Nervous System
Nervous System
The nervous system is an organ system containing a network of specialized cells called neurons that coordinate the actions of an animal and transmit signals between different parts of its body. In most animals the nervous system consists of two parts, central and peripheral. The central nervous system contains the brain, spinal cord, and retina. The peripheral nervous system consists of sensory neurons, clusters of neurons called ganglia, and nerves connecting them to each other and to the central nervous system. These regions are all interconnected by means of complex neural pathways. The enteric nervous system, a subsystem of the peripheral nervous system, has the capacity, even when severed from the rest of the nervous system through its primary connection by the vagus nerve, to function independently in controlling the gastrointestinal system.

Neurons send signals to other cells as electrochemical waves travelling along thin fibres called axons, which cause chemicals called neurotransmitters to be released at junctions called synapses. A cell that receives a synaptic signal may be excited, inhibited, or otherwise modulated. Sensory neurons are activated by physical stimuli impinging on them, and send signals that inform the central nervous system of the state of the body and the external environment. Motor neurons, situated either in the central nervous system or in peripheral ganglia, connect the nervous system to muscles or other effector organs. Central neurons, which in vertebrates greatly outnumber the other types, make all of their input and output connections with other neurons. The interactions of all these types of neurons form neural circuits that generate an organism’s perception of the world and determine its behavior. Along with neurons, the nervous system contains other specialized cells called glial cells (or simply glia), which provide structural and metabolic support.

Nervous systems are found in most multicellular animals, but vary greatly in complexity.[1] Sponges have no nervous system, although they have homologs of many genes that play crucial roles in nervous system function, and are capable of several whole-body responses, including a primitive form of locomotion. Placozoans and mesozoans—other simple animals that are not classified as part of the subkingdom Eumetazoa—also have no nervous system. In Radiata (radially symmetric animals such as jellyfish) the nervous system consists of a simple nerve net. Bilateria, which include the great majority of vertebrates and invertebrates, all have a nervous system containing a brain, spinal cord, and peripheral nerves. The size of the bilaterian nervous system ranges from a few hundred cells in the simplest worms, to on the order of 100 billion cells in humans. Neuroscience is the study of the nervous system.

Endocrine System
In animal anatomy the endocrine system is a system of glands, each of which secretes a type of hormone into the bloodstream to regulate the body. The endocrine system is an information signal system like the nervous system. Hormones regulate many functions of an organism, including mood, growth and development, tissue function, and metabolism. The field of study that deals with disorders of endocrine glands is endocrinology, a branch of internal medicine.

The endocrine system is made up of a series of ductless glands that produce chemicals called hormones. A number of glands that signal each other in sequence is usually referred to as an axis, for example, the hypothalamic-pituitary-adrenal axis. Typical endocrine glands are the pituitary, thyroid, and adrenal glands. Features of endocrine glands are, in general, their ductless nature, their vascularity, and usually the presence of intracellular vacuoles or granules storing their hormones. In contrast, exocrine glands, such as salivary glands, sweat glands, and glands within the gastrointestinal tract, tend to be much less vascular and have ducts or a hollow lumen.

In addition to the specialised endocrine organs mentioned above, many other organs that are part of other body systems, such as the kidney, liver, heart and gonads, have secondary endocrine functions. For example the kidney secretes endocrine hormones such as erythropoietin and renin.

Circulatory System:
The human circulatory system functions to transport blood and oxygen from the lungs to the various tissues of the body. The heart pumps the blood throughout the body. The lymphatic system is an extension of the human circulatory system that includes cell-mediated and antibody-mediated immune systems. The components of the human circulatory system include the heart, blood, red and white blood cells, platelets, and the lymphatic system.

Respiratory System:
Respiratory System:
The respiratory system’s function is to allow oxygen exchange through all parts of the body. The space between the alveoli and the capillaries, the anatomy or structure of the exchange system, and the precise physiological uses of the exchanged gases vary depending on organism. In humans and other mammals, for example, the anatomical features of the respiratory system include airways, lungs, and the respiratory muscles. Molecules of oxygen and carbon dioxide are passively exchanged, by diffusion, between the gaseous external environment and the blood. This exchange process occurs in the alveolar region of the lungs.[1]

Other animals, such as insects, have respiratory systems with very simple anatomical features, and in amphibians even the skin plays a vital role in gas exchange. Plants also have respiratory systems but the directionality of gas exchange can be opposite to that in animals. The respiratory system in plants also includes anatomical features such as holes on the undersides of leaves known as stomata.

Digestive System:
Digestive System:
The human digestive system is a complex series of organs and glands that processes food. In order to use the food we eat, our body has to break the food down into smaller molecules that it can process; it also has to excrete waste.

Most of the digestive organs (like the stomach and intestines) are tube-like and contain the food as it makes its way through the body. The digestive system is essentially a long, twisting tube that runs from the mouth to the anus, plus a few other organs (like the liver and pancreas) that produce or store digestive chemicals.

Urinary System:
Urinary System:
Your body takes nutrients from food and uses them to maintain all bodily functions including energy and self-repair. After your body has taken what it needs from the food, waste products are left behind in the blood and in the bowel. The urinary system works with the lungs, skin, and intestines—all of which also excrete wastes—to keep the chemicals and water in your body balanced. Adults eliminate about a quart and a half of urine each day. The amount depends on many factors, especially the amounts of fluid and food a person consumes and how much fluid is lost through sweat and breathing. Certain types of medications can also affect the amount of urine eliminated.

The urinary system removes a type of waste called urea from your blood. Urea is produced when foods containing protein, such as meat, poultry, and certain vegetables, are broken down in the body. Urea is carried in the bloodstream to the kidneys.

The kidneys are bean-shaped organs about the size of your fists. They are near the middle of the back, just below the rib cage. The kidneys remove urea from the blood through tiny filtering units called nephrons. Each nephron consists of a ball formed of small blood capillaries, called a glomerulus, and a small tube called a renal tubule. Urea, together with water and other waste substances, forms the urine as it passes through the nephrons and down the renal tubules of the kidney.

From the kidneys, urine travels down two thin tubes called ureters to the bladder. The ureters are about 8 to 10 inches long. Muscles in the ureter walls constantly tighten and relax to force urine downward away from the kidneys. If urine is allowed to stand still, or back up, a kidney infection can develop. Small amounts of urine are emptied into the bladder from the ureters about every 10 to 15 seconds.

The bladder is a hollow muscular organ shaped like a balloon. It sits in your pelvis and is held in place by ligaments attached to other organs and the pelvic bones. The bladder stores urine until you are ready to go to the bathroom to empty it. It swells into a round shape when it is full and gets smaller when empty. If the urinary system is healthy, the bladder can hold up to 16 ounces (2 cups) of urine comfortably for 2 to 5 hours.

Circular muscles called sphincters help keep urine from leaking. The sphincter muscles close tightly like a rubber band around the opening of the bladder into the urethra, the tube that allows urine to pass outside the body.

Nerves in the bladder tell you when it is time to urinate, or empty your bladder. As the bladder first fills with urine, you may notice a feeling that you need to urinate. The sensation to urinate becomes stronger as the bladder continues to fill and reaches its limit. At that point, nerves from the bladder send a message to the brain that the bladder is full, and your urge to empty your bladder intensifies.

When you urinate, the brain signals the bladder muscles to tighten, squeezing urine out of the bladder. At the same time, the brain signals the sphincter muscles to relax. As these muscles relax, urine exits the bladder through the urethra. When all the signals occur in the correct order, normal urination occurs.

Reproductive System:
Sexual reproduction is the process of producing offspring for the survival of the species, and passing on hereditary traits from one generation to the next. The male and female reproductive systems contribute to the events leading to fertilization. Then, the female organs assume responsibility for the developing human, birth, and nursing. The male and female gonads (testes and ovaries) produce sex cells (ova and sperm) and the hormones necessary for the proper development, maintenance, and functioning of the organs of reproduction and other organs and tissues. The reproductive system comprises the reproductive organs. In the male, the organs include the testes, accessory ducts, accessory glands, and penis. In the female, the organs include the uterus, uterine tubes, ovaries, vagina, and vulva.

Male Reproductive System:
Male Reproductive System:
The human male reproductive system (or male genital system) consists of a number of sex organs that are a part of the human reproductive process. In the case of men, these sex organs are located outside a man’s body, around the pelvic region.

The main male sex organs are the penis and the testes which produce semen and sperm, which as part of sexual intercourse fertilize an ovum in a woman’s body and the fertilized ovum (zygote) gradually develops into a fetus, which is later born as a child

Female Reproductive System:
Female Reproductive System:
The female reproductive system (or female genital system) contains two main parts: the uterus, which hosts the developing fetus, produces vaginal and uterine secretions, and passes the male’s sperm through to the fallopian tubes; and the ovaries, which produce the female’s egg cells. These parts are internal; the vagina meets the external organs at the vulva, which includes the labia, clitoris and urethra. The vagina is attached to the uterus through the cervix, while the uterus is attached to the ovaries via the Fallopian tubes. At certain intervals, the ovaries release an ovum, which passes through the Fallopian tube into the uterus.

If, in this transit, it meets with sperm, the sperm penetrate and merge with the egg, fertilizing it. The fertilization usually occurs in the oviducts, but can happen in the uterus itself. The zygote then implants itself in the wall of the uterus, where it begins the processes of embryogenesis and morphogenesis. When developed enough to survive outside the womb, the cervix dilates and contractions of the uterus propel the fetus through the birth canal, which is the vagina.

The ova are larger than sperm and have formed by the time a female is born. Approximately every month, a process of oogenesis matures one ovum to be sent down the Fallopian tube attached to its ovary in anticipation of fertilization. If not fertilized, this egg is flushed out of the system through menstruation.