592
UNIT 3
Regulation and Integration of the Body
16
Te endocrine glands include the pituitary, thyroid, parathy-
roid, adrenal, and pineal glands
(Figure 16.1)
. Te hypotha-
lamus, along with its neural functions, produces and releases
hormones, so we can consider the hypothalamus a
neuroendo-
crine organ
. In addition, several organs, such as the pancreas,
gonads (ovaries and testes), and placenta, contain endocrine tis-
sue and also perform other functions.
Many other organs also contain scattered endocrine cells
or small clusters of endocrine cells. For example, adipose cells
release leptin, the thymus releases thymic hormones, and
hormone-producing cells are found in the walls of the small in-
testine, stomach, kidneys, and heart—organs whose chief func-
tions have little to do with hormone production. We describe
these other hormone-producing structures on pp. 621–622.
Some physiologists include local chemical messengers—
autocrines and paracrines—as part of the endocrine system, but
that is not the consensus. Hormones are long-distance chemical
signals that travel in blood or lymph throughout the body. Au-
tocrines and paracrines, on the other hand, are short-distance
signals.
Autocrines
are chemicals that exert their effects on
the same cells that secrete them. For example, certain prosta-
glandins released by smooth muscle cells cause those smooth
muscle cells to contract.
Paracrines
also act locally (within the
The Endocrine System:
An Overview
Indicate important differences between hormonal and
neural controls of body functioning.
List the major endocrine organs, and describe their body
locations.
Distinguish between hormones, paracrines, and autocrines.
As we have seen, the nervous system regulates the activity of
muscles and glands via electrochemical impulses delivered by
neurons, and those organs respond within milliseconds. Te
means of control and speed of the endocrine system are very
different: Te endocrine system influences metabolic activity
by means of
hormones
(
hormone
5
to excite). Hormones are
chemical messengers secreted by cells into the extracellular
fluids. Tese messengers travel through the blood and regulate
the metabolic function of other cells in the body. Binding of
a hormone to cellular receptors initiates responses that typi-
cally occur a±er a lag period of seconds or even days. But, once
initiated, those responses tend to last much longer than those
induced by the nervous system.
Hormones ultimately target most cells of the body, produc-
ing widespread and diverse effects. Te major processes that
these “mighty molecules” control and integrate include:
Reproduction
Growth and development
Maintenance of electrolyte, water, and nutrient balance of the
blood
Regulation of cellular metabolism and energy balance
Mobilization of body defenses
As you can see, the endocrine system orchestrates processes that
go on for relatively long periods, in some instances continu-
ously. Te scientific study of hormones and the endocrine or-
gans is called
endocrinology
.
Compared with other organs of the body, those of the endo-
crine system are small and unimpressive. Indeed, to collect 1 kg
(2.2 lb) of hormone-producing tissue, you would need to collect
all the endocrine tissue from eight or nine adults! Unlike most
organ systems, the endocrine organs are not grouped together
but are widely scattered about the body.
As we explained in Chapter 4, there are two kinds of glands:
Exocrine glands
produce nonhormonal substances, such as
sweat and saliva, and have ducts that carry these substances
to a membrane surface.
Endocrine glands
, also called
ductless glands
, produce hor-
mones and lack ducts. Tey release their hormones into the
surrounding tissue fluid (
endo
5
within;
crine
5
to secrete),
and they typically have a rich vascular and lymphatic drain-
age that receives their hormones. Most of the hormone-
producing cells in endocrine glands are arranged in cords
and branching networks—a situation that maximizes con-
tact between them and the capillaries surrounding them.
Pineal gland
Hypothalamus
Pituitary gland
Parathyroid glands
(on dorsal aspect
of thyroid gland)
Thymus
Thyroid gland
Adrenal glands
Pancreas
Gonads
Ovary (female)
Testis (male)
Figure 16.1
Location of selected endocrine organs of the
body.
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