Anatomy and Physiology: Hormones
When you are a single cell, you can easily control what you are doing, but things get awfully complicated when you are multicellular. It is very important that the body be able to control the various goings-on in the tissues. This is especially important as the organs and body systems do not function in isolation; each depends on the others for both general needs (such as oxygen) and specific needs (such as when the gall bladder releases bile after eating fat).
The body has two ways of controlling the body tissues. One is through the nervous system, with its miles and miles of neural pathways. This type of control has the advantage of being very rapid, as you know if you have ever put your hand on a hot burner! The other, slower way involves hormones, which are a collection of chemicals that alter the behavior of specific organs and tissues on the cellular level. This type of control system relies on the blood to disseminate its chemical messages. Welcome to the glory that is the endocrine system.
Excerpted from The Complete Idiot's Guide to Anatomy and Physiology 2004 by Michael J. Vieira Lazaroff. All rights reserved including the right of reproduction in whole or in part in any form. Used by arrangement with Alpha Books, a member of Penguin Group (USA) Inc.