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. The strongest stimulus they can tolerate may be more than a trillion times as intense as the weakest they can detect. Still, there are many kinds of signals in the world that people cannot detect directly. Attending closely to any one input of information usually reduces the ability to attend to others at the same time.