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Chapter 11
t
he endocrine system, along with the nervous system, is one of the body’s two major communication
systems. Communication within the nervous system is rapid, whereas the signals sent by the endocrine
system may have much longer delays and last for much greater lengths of time. The
endocrine system
consists of all those glands, called
endocrine glands,
that secrete hormones.
Hormones
are chemical
messengers that enter the blood, which carries them from endocrine glands to the cells upon which they act.
The cells a particular hormone infl uences are the target cells for that hormone.
Endocrine glands are distinguished from another type of gland in the body, called exocrine glands.
Endocrine glands release their secretory products into the blood; exocrine glands secrete their products
into a duct, from where the secretions either exit the body (as in sweat) or enter the lumen of another
organ, such as the intestines (
Figure 11–1
).
Table 11–1
summarizes most of the endocrine glands, the hormones they secrete, and the major
functions the hormones control. The endocrine system differs from most of the other organ systems of
the body in that the various glands are not anatomically connected; however, they do form a system in
the functional sense. The reader may be puzzled to see some organs—the heart, for instance—that clearly
have other functions yet are listed as part of the endocrine system. The explanation is that, in addition to
the cells that carry out the organ’s other functions, the organ also contains cells that secrete hormones.
This illustrates the fact that organs are made up of different types of cells.
Note also in Table 11–1 that the hypothalamus, a part of the brain, is considered part of the endocrine system.
This is because the chemical messengers released by certain neuron terminals in both the hypothalamus and
its extension, the posterior pituitary, do not function as neurotransmitters affecting adjacent cells, but rather
enter the blood as neurohormones. The blood then carries them to their sites of action.
Table 11–1 demonstrates that there are a large number of endocrine glands and hormones. This chapter
is not meant to be all-inclusive. Some of the hormones listed in Table 11–1 are best considered in the
context of the control systems in which they participate. For example, the pancreatic hormones (insulin
and glucagon) are described in Chapter 16, on organic metabolism.
One phenomenon evident from Table 11–1 is that a single gland may secrete multiple hormones. The
usual pattern in such cases is that a single cell type secretes only one hormone, so that multiple hormone
secretion refl ects the presence of different types of endocrine cells in the same gland. In a few cases,
however, a single cell may secrete more than one hormone.
Finally, a chemical messenger secreted by an endocrine gland cell may also be secreted by other cell types
and serves in these other locations as a neurotransmitter or paracrine/autocrine agent. For example,
prolactin is secreted not only by the anterior pituitary but by cells of the immune system, where it is
thought to exert paracrine/autocrine functions. Somatostatin, a hormone produced by the hypothalamus,
is also secreted by cells of the stomach, where it has local paracrine actions.
The aims of this chapter are to present (1) the general principles of endocrinology—that is, a structural
and functional analysis of hormones in general that transcends individual glands; and (2) an analysis of
several of the most important hormonal systems. The reader is advised to review the material in Chapter 5
before continuing.
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