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
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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 tissue
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is 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 activity.
Protein hormones react with receptors on the surface of the
cell, and the sequence of events that results in hormone action
is relatively rapid. Steroid hormones typically react with
receptor sites inside a cell. Because this method of action
actually involves synthesis of proteins, it is relatively
slow.
Control of Hormone Action
Hormones are very potent substances, which means that very
small amounts of a hormone may have profound effects on metabolic
processes. Because of their potency, hormone secretion must
be regulated within very narrow limits in order to maintain
homeostasis in the body.
Many hormones are controlled by some form of a negative feedback
mechanism. In this type of system, a gland is sensitive to
the concentration of a substance that it regulates. A negative
feedback system causes a reversal of increases and decreases
in body conditions in order to maintain a state of stability
or homeostasis. Some endocrine glands secrete hormones in
response to other hormones. The hormones that cause secretion
of other hormones are called tropic hormones. A hormone from
gland A causes gland B to secrete its hormone. A third method
of regulating hormone secretion is by direct nervous stimulation.
A nerve stimulus causes gland A to secrete its hormone.
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