The endocrine system

Typically, the nervous system is adapted to convey messages rapidly between specific locations so that quick responses can be made. In contrast, the endocrine systemis adapted to carry information from one source to many destinations to bring about long-lasting responses.

The endocrine system consists of a number of glands that secrete hormones(organic chemicals, usually proteins or steroids). The glands of the endocrine system are called endocrine glandsor ductless glandsbecause they secrete their hormones directly into the bloodstream. Once inside a blood vessel, a hormone is carried in the bloodstream so that it can reach almost any cell in the body. However, each hormone has its own target cellson which it acts. Therefore, although all the hormones are transported together in the bloodstream, each has its own specific effect on the body. In some cases, a target cell has specific receptor molecules on its cell surface membrane which bind the hormone molecule. Once bound onto the membrane, the hormone brings about its response.

Endocrine glands occur at strategic points around the body. Their hormones regulate a wide range of activities, including blood glucose concentration, gastric secretion, heart rate, metabolism, growth rate, reproduction, and water balance.