Introduction to the Endocrine System
The endocrine system, along with the nervous system, functions in
the regulation of body activities. The nervous system acts through
electrical impulses and neurotransmitters to cause muscle
contraction and glandular secretion. The effect is of short duration,
measured in seconds, and localized. The endocrine system acts
through chemical messengers called hormones that influence
growth, development, and metabolic activities. The action of the
endocrine system is measured in minutes, hours, or weeks and is
more generalized than the action of the nervous system.
There are two major categories of glands in the body - exocrine and endocrine.
Exocrine glands have ducts that carry their secretory product to a surface. These glands
include the sweat, sebaceous, and mammary glands and, the glands that secrete digestive
The endocrine glands do not have ducts to carry their product to a surface. They are
called ductless glands. The word endocrine is derived from the Greek terms "endo,"
meaning within, and "krine," meaning to separate or secrete. The secretory products of
endocrine glands are called hormones and are secreted directly into the blood and then
carried throughout the body where they influence only those cells that have receptor sites
for that hormone.
Characteristics of Hormones
Chemical Nature of Hormones
Chemically, hormones may be classified as either proteins or steroids. All of the
hormones in the human body, except the sex hormones and those from the adrenal cortex,
are proteins or protein derivatives.
Mechanism of Hormone
Action Hormones are carried by the blood throughout the entire body, yet they affect
only certain cells. The specific cells that respond to a given hormone have receptor sites
for that hormone. This is sort of a lock and key mechanism. If the key fits the lock, then
the door will open. If a hormone fits the receptor site, then there will be an effect. If a
hormone and a receptor site do not match, then there is no reaction. All the cells that have
receptor sites for a given hormone make up the target tissue for that hormone. In some
cases, the target tissueis localized in a single gland or organ. In other cases, the target
tissue is diffuse and scattered throughout the body so that many areas are affected.
Hormones bring about their characteristic effects on target cells by modifying cellular