A hormone is a chemical that is produced in one tissue and transported via the circulatory system to a different target tissue. There, it causes a physiological change in the target.

Hormones are the chemical messengers of the endocrine system. The endocrine system also includes the ductless glands that synthesize and secrete hormones, and incorporates the responding target cells as well. Hormones are secreted by endocrine glands directly into the circulatory system, from which they contact nearly all cells of the body. Some endocrine glands, such as the adrenal glands, form organs of their own, while others are just parts of organs. The brain, for example, performs certain critical endocrine functions.

The endocrine system is one of two physiological systems responsible for the control of all biological processes. The other is the nervous system. While the nervous system controls specific, rapid biological responses, often to external stimuli, endocrine...