Because second messengers elicit a multistep pro-
cess, their effects are amplified. This means that a small
amount of nonsteroid hormone can produce an exagger-
ated and greatly diversified set of responses in the tar-
get cells. Note that this amplification occurs in each step
depicted in Figure 9.3: One hormone molecule can elicit
changes in many proteins. Also, because nonsteroid hor-
mones do not involve making new proteins, they generally
act faster, but their effects are more short-lived than those
of steroid hormones.
A variety of endocrine cells make nonsteroid hor-
mones that act on many target cells. For example, the
adrenal medulla makes epinephrine and norepinephrine,
which are also called adrenaline and noradrenaline, re-
spectively. These hormones increase heart rate, contract
blood vessels to increase blood pressure, and stimulate
the liver to break down glycogen into glucose. The hy-
pothalamus makes antidiuretic hormone (ADH), which
is released from the pituitary gland and acts on the kid-
neys to reabsorb water. The pituitary gland also produces
follicle-stimulating hormone (FSH), which stimulates sex
cell production in the ovaries and testes. The pancreas se-
cretes insulin, which acts on most cells in the body to take
up glucose and store lipids.
1.
How
does an endocrine gland communicate
with a target cell?
2.
How
does a steroid hormone exert its action?
3.
What
is the role of the second messenger?
Endocrine Glands Regulate
Key Body Functions
LEARNING OBJECTIVES
1.
Identify
the hormones secreted by the pituitary
gland and their physiological effects.
2.
Outline
the processes of secretion and the
functions of thyroid hormones.
he endocrine glands, organs containing en-
docrine cells, and endocrine tissues secrete
approximately 30 different hormones. They
regulate and control many body functions,
including chemical composition and volume of blood, me-
tabolism, contractions of smooth and heart muscles, se-
cretions of endocrine glands, the immune system, growth
and development, reproduction, and daily rhythms (cir-
cadian rhythms).
Diseases or endocrine disorders may involve dimin-
ished secretions of endocrine glands (hyposecretion) or
increased secretions of endocrine glands (hypersecretion).
Some hormone secretions are associated with more than
3.
Explain
the roles of parathyroid glands in regu-
lating calcium metabolism.
4.
Describe
the role of the pancreatic islets in glu-
cose homeostasis.
one endocrine gland, as you will see with the hypothalamic-
pituitary-thyroid axis. Diminished hormonal secretion by
an endocrine gland may have one of two causes:
P rim ary hyposecretion.
This is a defect in a gland that directly
secretes a hormone.
Secondary hyposecretion.
This is a defect in a gland that
provides a stimulating hormone or releasing hormone
to the gland that directly secretes a hormone.
Let’s look at the functions of four major elements of
the endocrine system: the hypothalamus/pituitary gland,
thyroid gland, parathyroid gland, and pancreas.
T
Endocrine Glands Regulate Key Body Functions
257
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