The endocrine system is in charge of regulating body activities by secreting hormones. Hormones are carried into the tissues and organs of the body by the bloodstream.

B. endocrine system

Cholesterol, which is a lipid, produces steroid hormones only.

B. steroid hormones

Prostaglandins are hormone-like substances that are produced when there is a chemical reaction at the site where the cell or tissue is affected; hence, the term “local hormones.” These substances cause contraction, pain, fever, inflammation, or clotting when there is a damaged tissue in the body.

B. prostaglandins

The parathyroid gland regulates calcium levels in the body.

D. parathyroid gland

Thyroid gland produces a hormone called thyroxine, which is responsible for regulating the body’s metabolism.

C. thyroxine

In the **female reproductive system**, the **egg cells** are produced in the **ovaries**, then they will be released and will travel through the **Fallopian tubes** that connect immediately to the uterus. The uterus is the place where the baby develops and it leads into the **vagina**, the opening outside of the female’s body. In the diagram below, the x is labeled in the uterus, hence the correct answer is **a**.

A

The testes in males produce the primary male sex hormones called testosterone.

B. testosterone

Once the sperm fertilizes with the egg cell, a zygote is formed.

C. zygote

The process of fertilization takes place inside the fallopian tube of female individuals.

C. Fallopian tube

During the development of an offspring, a fetus forms after a period of eight weeks. During this time, the umbilical cord is formed too.

C. fetus

**Hormones** are chemical signals that are released from the endocrine glands and function in the maintenance of homeostasis, growth, development, and sexual characteristics. Hormones travel in the bloodstream and affect the processes in their **target cells**.

**Target cells** are specific cells that contain a particular receptor for a given hormone. Hence, if a cell does not have a receptor for a given hormone, the hormone will not act upon it. This type of scenario is similar to the lock-and-key principle of an **enzyme-substrate** complex. Where only a specific substrate can form an attachment with the active site of an enzyme.

Prostaglandins are hormone-like substances that are produced when there is a chemical reaction at the site where the cell or tissue is affected; hence, the term “local hormones.” These substances cause contraction, pain, fever, inflammation, or clotting when there is a damaged cell or tissue in the body.

Once a stimulus is detected, the endocrine system releases signals, which are influenced by feedback mechanisms, to regulate the activities of the cells in our body. This feedback mechanism either tells a gland to start producing or stop producing hormones in the bloodstream.

For example, if the body has detected a stimulus, which in this case is a high blood sugar level in the body, the endocrine system gives a signal to the pancreas to produce insulin. Insulin is a hormone that regulates the blood sugar level in the body.

The **pituitary gland** is a small pea-sized endocrine gland that is commonly referred to as the “master gland”. This gland is composed of two lobes (posterior and anterior lobes) that regulate and control many bodily processes through the various hormones it produces and secretes.

The **anterior lobe** also referred to as **adenohypophysis** of the pituitary gland caters to almost 80% of this gland. This lobe secretes hormones like adrenocorticotropic hormone, **growth hormone (GH)**, luteinizing hormone, follicle-stimulating hormone, prolactin, and thyroid-stimulating hormone (TSH).

The **endocrine glands** produce and release hormones that regulate many body functions. During times of stress, the sympathetic nervous system coordinates with the endocrine system to create hormones that will signal the adrenal glands to increase the production of two important stress hormones: **epinephrine** and **norepinephrine**.

**Epinephrine** is considered more powerful compared to norepinephrine. it produces a response that causes stimulation to the body which results in an increase in heart rate, blood pressure, and blood flow in the muscles. Moreover, it also increases the dilatation of the air passage in order for the body to take more oxygen and also releases extra glucose molecules in the blood for a sudden need of energy.

The **pancreas** is a type of gland that functions both as exocrine and endocrine. It produces enzymes for the digestion process as well as hormones that regulate blood sugar levels. The two types of hormones under the regulation of the pancreas are **insulin** and **glucagon**.

**Insulin** and **glucagon** maintain the stability of glucose in the blood because if there is a large amount of glucose moving in the blood it will cause a disease called **diabetes mellitus** and if there is a few or low amount of glucose in the blood there will not be enough energy for the body to function.

**Puberty** is the period or stage of rapid growth and maturation of the sex characteristics of an individual. This is the period in which the reproduction system is fully developed and functional. Puberty usually occurs any time between the age of 9 to 15, and on average, females undergo this period one year earlier than males.

**Puberty** is the period or stage of rapid growth and maturation of the sex characteristics of an individual. This is the period in which the reproduction system is fully developed and functional. Puberty usually occurs any time between the age of 9 to 15, and on average, females undergo this period one year earlier than males.

**Puberty** starts when the hypothalamus sends chemical signals to the pituitary gland to increase the production and secretion of two hormones– **follicle-stimulating hormone (FSH)** and **luteinizing hormone (LH)** that affect the growth, development, and functions of the gonads.

**Adolescence** is the beginning of puberty and the end of the period of childhood. During this stage, hormones such as HCG, thyroxine, and steroids will be more active–aiding in the rapid growth and development of sexual characteristics of an individual.

During male puberty, the individual will grow approximately 7-13cm in one year and his shoulders become broader. His voice will mature, and there will be a growth and increase in his body and facial hair. The testes will then begin to produce the hormone testosterone.

The **sperm** is a specialized cell that functions in reproduction. The sperm is composed of a **head** that contains the nucleus and chromosomes, a **midpiece** that contains many mitochondria that help in the energy supply of the cell, a **tail** or **flagellum** that helps in the propulsion of the cell as it travels in the female reproductive tract.

The sperm is a specialized cell that is produced in the testes and functions in the reproduction of the human species. In the male reproductive system, the **sperm cells** are produced in the testes through the seminferous tubules. The sperm then travels to the epididymis where it is stored before it matures. If the sperm is ready to release from the body, it then passes through vas deferens then to the urethra .

There are two types of hormones that are made in the ovaries and they are **estrogen** and **progesterone**. These two are also regulated by the LH and FSH in a negative feedback manner which helps in the production of egg cells.

The hormone **estrogen** specifically functions in the regulation of growth and development of secondary female characteristics and as well as controls other physiological body processes needed in reproduction. One example of this is during the follicular phase of the menstrual cycle. Estrogen causes the lining of the uterus to thicken and prepare the body to receive the fertilized egg.

In the **menstrual cycle**, the egg in the woman’s body develops and is released from the ovary for fertilization. If the egg is fertilized, it will be developed into a zygote, but if it is not fertilized, it will be discharged outside the body. There are four phases in this cycle: the follicular phase, ovulation phase, luteal phase, and menstruation.

**Four phases of the menstrual cycle:**

1. Follicular phase – The hypothalamus creates a response and stimulates the anterior pituitary gland to release FSH and LH in the blood in order for the follicles to develop to maturity.

2. Ovulation phase – This is the shortest among the phases, as it only lasts for three to four days. In this phase, the matured egg is released into one of the Fallopian tubes, ready for fertilization.

3. Luteal phase – This is the phase after the egg is released into the Fallopian tubes and it will turn into a yellow-colored structure known as corpus luteum. The corpus luteum will continue to secrete estrogen, and progesterone helps in the stimulation of cell growth in the uterus lining.

4. Menstruation – This is the phase after the egg was not fertilized. In this phase, the lining of the uterus will shred and detach along with the blood and unfertilized egg, and it will be discharged outside through the vagina. Menstruation lasts for an average of 3-5 days, and a new cycle will begin after this phase.

The **menstrual cycle** is an example of a feedback mechanism because of its ability to move from one phase to another only when certain hormones become too low or too high. When **estrogen is too low**, the **anterior pituitary gland begins producing LH and FSH that stimulates the growth of an egg cell which then helps increase the estrogen levels**.

**Fertilization** is the process in which the egg cell and the sperm unite. This can happen during sexual intercourse in which the sperm are released into the vagina during ejaculation. The sperm swim actively into the Fallopian tube, and a successful sperm will penetrate the egg. After penetration, the nucleus and chromosomes of the sperm cell will unite with that of the egg cell, hence finishing the fertilization process. The fertilized egg now called a **zygote** contains a set of chromosomes from each parent

The **zygote** will begin to develop and continue to undergo mitosis (cell division) as it travels from the Fallopian tube to the uterine wall, where it attaches itself for further embryonic development. The period that caters to this maturation is called *early development*.

The early development stage is composed of three phases called implantation, gastrulation, and neurulation. **Implantation** is the phase in which the fertilized egg implants itself in the uterine wall, six or seven days after fertilization. In this phase, the differentiation process starts to occur. Differentiation is the process in which the cells begin to develop specialized tissues for the development of the body.

**Primary germ layers** are groups of cells that form during the formation of an embryo. A human embryo produces three germ layers: **the ectoderm, the endoderm, and the mesoderm**. Each of these germ layers gives rise to different types of tissues. These germ layers are important because without them the embryo would not be able to develop properly.

The **placenta** is an organ that is formed during pregnancy which functions to provide oxygen and nutrients to the developing embryo. It also removes waste products from the blood of the baby. The placenta serves as a connection between the mother and the baby.

**Childbirth** is the end stage of pregnancy in which the baby exit from the mother’s internal environment via the process of delivery. There are various factors that affect childbirth and delivery including the production and release of specific hormones.

The hormone *oxytocin* plays an important role in childbirth. It is a hormone released from the pituitary gland, and it functions to stimulate a series of contractions in the involuntary muscles of the uterine wall. These contractions will be more frequent and powerful as it forcibly pushes the baby forward and out in the opening of the cervix through the vagina.

**Diabetes mellitus** is a disorder in which the pancreas organ fails to create, and secrete insulin thus resulting in an elevated amount of sugar in the blood. Insulin is a hormone needed for the absorption of sugar in the blood. Usually, insulin works after a person consume his or her meal and this can be observed as the amount of glucose in the blood will lessen after a period of time (1-2 hours after a meal)

Based on the given graph, the person with diabetes is indicated by the **red line**, and a person without diabetes is represented by a **blue line**. This is because it was noted that in the red line the amount of sugar in the blood is still at its peak even if it was already two hours after the person’s meal

The diagram below shows the negative feedback mechanism in the regulation of the thyroxine hormones in the body.

Click to see the diagram

A swimmer’s heartbeat increases before a competition are most likely due to the presence of the hormone **epinephrine** which results in a “fight or flight” response. This is because the swimmer is most likely experiencing excitement or nervousness right before the competition. The swimmer’s heart rate increases during the competition because of the physical demands of swimming.

The placenta is formed from both the embryo and the mother. Each part formed sit side by side (on a microscopic level) without direct vessel connection. This way the blood of the mother and the embryo do not mix.

See solution explanation.

The event is called **ectopic pregnancy**, and it is dangerous to the mother because when the embryo begins to grow it will eventually become too big for the fallopian tube, the increased size of the embryo might cause the tube to rupture thereby causing a life-threatening scenario.

See the explanation

**Puberty** is the period or stage of rapid growth and maturation of the sex characteristics of an individual. This is the period in which the reproduction system is fully developed and functional. Puberty usually occurs any time between the age of 9 to 15, and on average, females undergo this period one year earlier than males.

**Puberty** starts when the hypothalamus sends chemical signals to the pituitary gland to increase the production and secretion of two hormones– follicle-stimulating hormone (FSH) and luteinizing hormone (LH) that affect the growth, development, and functions of the gonads.

Therefore, if there is a low level of FSH and LH in a female individual, it may result in a negative effect on the various phases of the menstrual cycle. It can specifically result in an abnormal or insufficient maturity of the follicles, and irregular menstruation.

The reports act as a feedback mechanism because when individuals hear about a traffic jam, they will try and find a way to avoid it thereby decreasing the number of individuals traveling along that particular road. The decrease in the number of cars will most likely clear the traffic jam, allowing for more cars to pass through once more.

**Seminal fluid** provides nutrients to the sperm and a buffer from the environment of the female reproductive system. It allows for the sperm to live long enough for them to fertilize an egg. The **production and release of millions of sperm** allow for an increase in the probability of the egg being fertilized. **Cilia in the fallopian tubes** help the egg travel far enough to get implanted with a sperm cell. The **long tail of the sperm** cell allows for it to be able to move the long distance necessary in order to fertilize the egg.

It is important for the success of a full-term pregnancy because toward the end of a menstrual cycle, the linings of the walls of the uterus detach and the blood vessels that formed a few days earlier will also deteriorate. If a woman is pregnant and her body went through this process, the placenta would detach from the uterus and the fetus would be unable to receive nutrients and oxygen and get rid of waste, and eventually, it would die thus resulting in unsuccessful pregnancy.