ABSTRACT
The endocrine system employs chemical messengers called hormones to influence and regulate cellular activity throughout the body. Although hormones are much slower method of communication than nerve impulses they are capable of influencing many cells simultaneously and can exert a much longer duration of action. This chapter produces more than 80 different hormones which govern and control every aspect of the human body to greater or lesser extent. Hormones are secreted into the blood by an endocrine gland or by specialised cells situated within an organ. Endocrine glands differ from exocrine glands in that they don't discharge their products into a duct or onto the surface of the skin. In the case of the endocrine system, it is the hypothalamus and pituitary gland that control and regulate the activity of most of the other endocrine glands. Hormones secreted by the anterior pituitary gland are often referred to as tropic hormones since they turn on/off endocrine glands elsewhere in the body.
