Our bodies are made up of eleven systems, each of which has been designed to fulfill different functions:
Digestive. This is the system that breaks down food and absorbs its nutrients. The digestive tract is a long system of tubes that run from the mouth to the anus. It includes the esophagus, stomach, small intestine, and large intestine. The liver and the pancreas manufacture special enzymes to help break down food.
Muscular. This system provides the body with movement. There are three types of muscle: skeletal, cardiac, and smooth. Skeletal muscles attach to bones, and are voluntary--they are consciously controlled by the nervous system. Cardiac muscles cause the heart to pump blood, and are involuntary--they contract automatically. Smooth muscles are also involuntary, and cause movement in other organs; these are the muscles that push the food that we eat down the esophagus and into the stomach.
Integumentary. The integumentary system includes skin, which provides sensory receptors and protects the body.
Excretory. This system gets rid of various body wastes. It includes the sweat glands (where the body excretes unnecessary salts), kidneys (which filter the blood), and urinary tract.
Reproductive. The reproductive system allows for the continuation of life. Gametes from the male's sperm and the female's egg combine to form a zygote--a unique combination of genes which no other human being has!
Circulatory. This is the system that our blood flows through. It carries nutrients and oxygen to all the organs of the body, and carries away wastes.
Respiratory. The respiratory system provides the body with oxygen, and it expels carbon dioxide from the body. The nasal passage, trachea, bronchial tubes, lungs, and alveoli are involved in this process. Inhaled oxygen is broken down in the alveoli (tiny air sacs in the lungs) and then passed into the capillaries, where it travels into the bloodstream. In the same way, carbon dioxide from the blood is passed back into the alveoli, and exhaled from the body.
Skeletal. The skeletal system provides structure for the body and protection for the body's internal organs. Bones, ligaments, joints, and the skull are all part of the skeleton.
Immune. The body's immune system fights disease. The lymphatic system is the main defense mechanism within this system. A liquid called lymph flows over the tissues and carries off harmful bacteria, which is filtered out in the lymph nodes. White blood cells, which are produced in our bones' marrow, are another important part of the body's defenses.
Nervous. The brain, spinal cord, and nerves work together to coordinate the body's actions. Our senses are also part of this system, allowing us to see, taste, smell, touch, hear, and feel pressure and pain.
Endocrine. The endocrine system is made up of the chemical messengers which control many bodily activities: hormones. Hormones initiate many bodily processes, such as reproduction, growth, and digestion.
Check out Body by Design for more in depth information about each of these systems, and look at medical school quality charts in Systems and Structures.