The cells, the tissue organs, and the biological functions in the human or animal body are regulated or influenced practically every day by two anatomical and physiological systems. These are the nervous system and the endocrine system. The nervous system coordinates rapid and precise responses to stimuli through a momentary change in electrical potential in a cell or tissue. This momentary change occurs when a cell or tissue has been activated by a stimulus, and is commonly referred to as action potential. The functions performed by the nervous system are much more immediate and rapid, such as the control of breathing and body movement. Further discussion of this energy concept and the nervous system is beyond the scope of this lecture. Students with an interest in this subject area are referred to the basic textbooks on general physiology or neurophysiology.

The endocrine network, on the other hand, maintains homeostasis of and long-term control in our body by means of chemical signals. It works in parallel with the nervous system to control many body functions along with homeostasis. Homeostasis is a physiological term used to define a biological entity’s ability or tendency to maintain a steady internal state.

The endocrine system is a collection of ductless glands that secrete chemical messages which are commonly referred to as hormones. These signals are passed through the blood to arrive at target cells which possess the appropriate receptor, much like a lock of definite shape to which only one or certain keys can fit. Exocrine glands that secrete substances that are passed outside the body are ducted structures, such as digestive glands, sweat glands, and salivary glands. These ducted glands are thus by definition not part of the endocrine network.