Endocrine System

The body has two communication systems to coordinate the activities of different tissues and organs. One communication system is the nervous system and the other is the endocrine system. The endocrine system is the network of glands and tissues that secrete hormones, chemical messengers produced in one tissue and carried by the blood to act on other parts of the body. Compared to the nervous system, the signals conveyed by the endocrine system take much more time. A nervous impulse is produced in a millisecond and travels anywhere in the body in less than a second. Hormones require time to be synthesized, can travel no more quickly than the blood can carry them, and often cause actions through inducing protein synthesis or transcription that require time. However, hormone signals will tend to be more long-lasting than nerve impulses. When the nerve impulse ends, a target such as skeletal muscle returns quickly to its starting state. When a hormone induces protein synthesis, the proteins remain long after the hormone is gone. Often the two systems work together. The endocrine glands, such as the pancreas or the adrenal cortex, can be the direct targets (effectors) of the autonomic nervous system. The hormone adrenaline acts in concert with the sympathetic nervous system to produce a set of results similar to those produced directly by sympathetic neurons.

Many in this class plan to major in psychology. Because of this, we will go into the endocrine system in more detail than the other systems. The action of hormones can have profound effects on psychology.

Endocrine glands secrete hormones directly into the bloodstream. This is in contrast to exocrine secretions that do not contain hormones and are released through ducts into a body compartment. An example of exocrine secretion is the secretion by the pancreas of digestive enzymes into the small intestine through the pancreatic duct. Both endocrine and exocrine functions can be found in the same organ. The pancreas simultaneously produces exocrine secretions like digestive enzymes and endocrine secretions like insulin and glucagon that are released into the blood to exert their effects throughout the body.

This animation (Audio - Important) describes the glands of the endocrine system.