The human body is the most wonderful, complex machine in the world. Your body can walk and run, climb and sit. It can see, hear, smell, taste, and feel. It can eat, talk, laugh, lift things, play games, think, and read. While these things are going on, you are breathing; there is blood flowing through your blood vessels; you are digesting food; and your body is doing a million other jobs at the same time! All through your life this marvelous machine is at work-even when you are sleeping.

These functions are performed by the various body systems:

	The circulatory system: Your circulatory system is make up of your heart, arteries, veins, and very thin blood vessels called capillaries.			The endocrine system: The endocrine system produces chemical messengers called hormones which help to control other body functions.
	The respiratory system: Oxygen enters your body through your nose or mouth, moves down your windpipe and into your lungs.			The muscular system: Almost half the body's weight is muscle. Muscles are the part of our body that allow us to move.
	The digestive system: Your mouth, esophagus, stomach, large and small intestines and other organs make up your digestive system.			The skeletal system: provides shape and form of our bodies, supports, protects, and allows bodily movement; it also produces blood for the body, and stores minerals.
	The nervous system: Your brain, your spinal cord, and the millions of nerve cells in your body make up your nervous system.			The excretory system: The excretory system is responsible for the elimination of wastes produced by homeostasis.
	The blood and immune system: The body's immune system includes several kinds of white cells in blood, body fluids, and lymph nodes.			The reproductive system: All living things, including humans, reproduce; it's one of the things that set us apart from nonliving matter.