The level of organization in a multicellular organism begins with the individual cells. These individual cells form a tissue, and the tissues make up an organ. A group of organs forms an organ system, and in turn, these organ systems make up an organism.

**Homeostasis** is the ability of an organism to regulate internal conditions at a constant rate despite changes of the external environment. This is accomplished by a constant monitoring and feedback system.

11 Human organ systems:

1. **Nervous system**- It is composed of the brain, spinal cord, and nerves. This system functions to control and regulate the body’s responses to internal and external stimuli. It also plays a role in the coordination of other organ systems.

2. **Integumentary system** – It is composed of our skin, hair, nails, and integumentary glands. This system works to protect our body as it serves to create a barrier against the harmful environment. It also functions to regulate our body temperature.

3. **Skeletal system** – It is mainly composed of our bones and cartilages which function to support our body and protect our internal organs. This system also serves as an attachment site of our muscle tissues.

4. **Muscular system** – composed of muscle tissues that work with the skeletal system in order to provide movement for the body. This system also plays a role in the circulation of blood and the transport of food materials in our digestive tract.

5. **Circulatory system** – The system that is comprised of our heart, and blood vessels. Its function is to transport oxygen and nutrients in the body through blood flow and fights infection through the synthesis of antibodies.

6. **Respiratory system** – Composed of the nose, pharynx, larynx, trachea, bronchioles, lungs, and alveoli. This system together with the circulatory system provides the body with the needed oxygen for cellular respiration. Moreover, it also removes waste materials (carbon dioxide) from the body.

7. **Digestive system** – The system is composed of the gastrointestinal tract and organs which serves to convert our food source into simpler molecules that can be utilized by the cells in our body for energy production.

8. **Excretory system** – The system that contains our functions to eliminate the waste materials in the body in order to maintain homeostasis. The organ present in this system is our kidneys.

9. **Endocrine system** – This is the system composed of glands, that function to produce and secrete hormones for the regulation of growth, development, and reproduction.

10. **Reproductive system** – Divided into male and female reproductive organs which serve to play an important role in *reproduction*– the creation of sex cells, and the development of an embryo.

11. **Lymphatic system** – A subsidiary of the circulatory system that aids the body to protect the body against harmful antigens. Moreover, it also collects excess fluid and returns it to the normal body system.

There are **four types** of tissues in the human body:

A. **Epithelium** (Absorption, selective permeability, and detection of sensations)

B. **Connective tissue** (protection and support)

C. **Muscles** (support and movement)

D. **Nervous tissue** (Sensation and response)

The nervous tissue is composed differently than other types of tissue. There is a main body and extensions (like axons) that are used to communicate with other nerve cells. The brain and spinal cord would contain these cells, as well as other parts of the body that the brain and spinal cord control and regulate