The Endocrine System
A hormone is a chemical messenger which is secreted by an endocrine gland into the bloodstream. It then travels by way of the blood to another part of the body and exerts an effect on a target tissue or organ. The messenger function of the endocrine system has been compared to that of the nervous system. However, a hormone takes longer to cause an effect than a nerve signal and its effects generally last longer.
The endocrine glands are the regulators of the body, controlling many bodily functions including drives, emotions, growth, energy production, and repair of damaged tissues. The secretion of hormones by the endocrine glands is regulated by a negative feedback cycle. An example will illustrate how this works:
Suppose you have just eaten a meal. The sugars from the meal have been absorbed from your small intestines into the bloodstream thus resulting in an increase in your blood sugar level. The pancreas senses this increase and secretes insulin into the bloodstream to bring the sugar level down by allowing it to move into the cells where it is needed for energy. Once your blood sugar level has been lowered, the pancreas responds to this (negative feedback) by decreasing its secretion of insulin.
The secretion of hormones is regulated primarily by. . .