Eva Fletcher, age 14, of Decatur, Ill., for her question

WHERE ARE HORMONES PRODUCED IN THE BODY?

Hormones are chemical substances that are produced in one part of an organism but cause effects in other parts. Thus, hormones serve as a means of communication among various parts of an organism.

In the human body, most hormones are produced by organs called endocrine, or ductless, glands. The hormones control such important body activities as growth and development.

The major endocrine glands include the two adrenal glands, the pituitary gland, the four parathyroid glands, the sex glands and the thyroid glands. A few hormones are produced by endocrine tissue present in organs that are not primarily endocrine glands. Such organs include the brain, the stomach and the pancreas.

The endocrine glands secrete hormones into the blood, which carries them throughout the body. After a hormone arrives at its "target," the organ or tissue it affects, it causes certain actions to occur.

Hormones regulate a variety of body functions. They may be grouped according to the functions they control. These functions include the way the body uses food, growth, sex and reproduction; the regulation of the composition of the blood; the body's reaction to emergencies: and the control of hormones themselves.

Hormones play a key role in regulating the body's development from infancy to adulthood.

Metabolic hormones regulate the various steps in metabolism, the process by which the body converts food into energy and living tissue. For example, the endocrine tissue of the stomach and small intestine secretes a number of digestive hormones that control the secretion of digestive juices.

The rate at which the cells of the body use food to release energy are controlled by two hormones produced by the thyroid gland: thyroxine and triiodothronine. Other hormones control the way cells use food to build new tissue.

In 1902 scientists found the first definite evidence of the existence of hormones. That year, British researchers discovered that a chemical substance controlled certain activities involved in digestion.

Since early in the century, scientists have identified more than 30 hormones produced by the human body. They also have developed ways of removing hormones from living tissue and techniques for manufacturing them in the laboratory.

It was in the 1940s that biochemists learned to synthesize or artificially create many hormones. Until then, almost all hormones had to be extracted from animals or plants. Hormones occur in extremely small quantities in living tissue.

Some synthetic hormones are exact duplicates of the natural secretions. But scientists have altered the chemical structure of many synthetic hormones to provide more powerful action. The most common man made hormones include auxins and glucocorticoids.