To burn food for the release of energy stored in it, oxygen must be supplied to cells, and carbon dioxide removed. Lungs take in oxygen for the combustion of food, and they eliminate the carbon dioxide produced. The urinary system disposes of dissolved waste molecules, the intestinal tract removes solid wastes, and the skin and lungs rid the body of heat energy. The circulatory system moves all these substances to or from cells where they are needed or produced, responding to changing demands. Specialized cells and the molecules they produce identify and destroy microbes that get inside the body.
Hormones are chemicals from glands that affect other body parts.
They are involved in helping the body respond to danger and in regulating human growth, development, and reproduction. Interactions among the senses, nerves, and brain make possible the learning that enables human beings to cope with changes in their environment. All behavior is affected by both inheritance and experience. The level of skill a person can reach in any particular activity depends on innate abilities, the amount of practice, and the use of appropriate learning technologies. Human beings can detect a tremendous range of visual and olfactory stimuli.