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Groups of cells form tissues in the body, and similar tissues form organs. Although organs act as individual units, they do not function independently; several combine to form a system, with each system having a special function.

A system is a combination of organs that performs a particular function. The systems of the body and their functions are:

Skeletal: framework of the body, supporting organs and furnishing a place of attachment for muscles. An adult skeleton consists of 206 bones divided into two groups: the axial skeleton and the appendicular skeleton. The skeleton is made of four different types of bone: (1) long bones, (2) short bones, (3) flat bones, and (4) irregular bones.

Bone is important in five ways. First, bones are the main support for the body; they give your body its shape. Without bones your body would lay flat like a puddle of water. For example, the bones that make up your spine allow you to sit up straight. The bones in your legs support the weight of your body so that you can stand up.

Another function of your skeleton is movement. Muscles are attached to bones and when muscles move, bones move. Third, bones protect the organs beneath them. The ribs provide protection for the lungs and heart. The skull protects the brain. Fourth, bone is a storage place for calcium and fat. Finally, bones are the location where blood cells are made.

Muscular: permits motions and movement of the body. Muscle is a tissue composed of fibres that can contract and relax to produce movement in a part of the body. We use muscles for all body movements, including walking, standing, typing on a keyboard, and chewing food. Even the beating of the heart is caused by muscles. There are three kinds of muscles: striated, smooth, and cardiac.

Integumentary: Integument is the word used within the medical profession for skin. The integument is the outer covering of the body. It holds all of the parts of the body inside and prevents unwanted things from getting into the body from the outside. It aids in temperature regulation and has functions in sensation and excretion. The integument has three layers or regions: epidermis, dermis, and hypodermis (fat). The integument also contains accessory structures such as blood vessels, nerves, nails, hair, oil glands and sweat glands.

Cardiovascular: transports the blood. It comprises the heart and blood vessels. Circulating the blood throughout the body, it carries essential supplies of food and fuel to every living cell and exchanges them for potentially harmful waste products. The blood is kept flowing round the body by the pumping action of the heart.

Lymphatic: retrieves plasma and tissue fluids and has a role in the immune system. It is a network of thin-walled vessels. They drain lymph from between the body cells into the blood stream. Lymph nodes (rounded bean-shaped structures) are also part of the lymphatic system.

Respiratory: absorbs oxygen and discharges carbon dioxide. This exchange of gases takes place in the lungs, which are the principle organs of breathing; the other organs of breathing are the nose, pharynx, larynx, trachea, and bronchi.

Digestive: The body needs energy to work properly. You need energy to move, study, and laugh. Sugars, protein, and fat in the foods you eat help your body obtain energy. The role of the digestive system is to take in food, break it down into small molecules, and put those molecules into the bloodstream to carry energy to all cells of the body. Cells use as much of the food energy as they need, and the rest either is stored as fat or eliminated (leaves the body).

The digestive system is comprised of the organs of the digestive tract (the path along which food travels) and accessory organs (the lips, teeth, tongue, salivary glands, hard and soft palates, liver, gallbladder and pancreas), which help with digestion but are not part of the path of food.

Genitourinary: reproduction and urine excretion. It includes the kidneys, ureters, bladder, and urethra in both sexes. In women, the ovaries, fallopian tubes, uterus, vagina, labia, and clitoris are also part of the GU system. In men, the testes, prostate gland, vas deferens, and penis are included in the GU system.

Endocrine: manufactures hormones. The chief endocrine gland is the pituitary. Other endocrine glands are the thyroid, adrenals, and sex glands.

Nervous with special senses: The nervous system is the main communication and decision-making system of the body. It processes stimuli and enables the body to act and respond. It is comprised of two main divisions: the central nervous system (CNS), which consists of the brain and the spinal cord; and the peripheral nervous system (PNS), which consists of spinal nerves and cranial nerves that link the CNS with the body's receptors and effectors.

Immune: protects the body from invading organisms and disease.