Essentials of Anatomy and Physiology

The Endocrine System 245

inside the cell, the steroid hormone combines with a
protein receptor in the cytoplasm, and this steroid-
protein complex enters the nucleus of the cell. Within
the nucleus, the steroid-protein complex activates spe-
cific genes, which begin the process of protein syn-
thesis. The enzymes produced bring about the cell’s
characteristic response to the hormone (see Fig.
10–12).

AGING AND THE

ENDOCRINE SYSTEM

Most of the endocrine glands decrease their secretions
with age, but normal aging usually does not lead to
serious hormone deficiencies. There are decreases in
adrenol cortical hormones, for example, but the levels
are usually sufficient to maintain homeostasis of water,
electrolytes, and nutrients. The decreased secretion of
growth hormone leads to a decrease in muscle mass

and an increase in fat storage. A lower basal metabolic

rate is common in elderly people as the thyroid slows

its secretion of thyroxine. Unless specific pathologies

develop, however, the endocrine system usually con-

tinues to function adequately in old age.

SUMMARY

The hormones of endocrine glands are involved in vir-

tually all aspects of normal body functioning. The

growth and repair of tissues, the utilization of food to

produce energy, responses to stress, the maintenance

of the proper levels and pH of body fluids, and the

continuance of the human species all depend on hor-

mones. Some of these topics will be discussed in later

chapters. As you might expect, you will be reading

about the functions of many of these hormones again

and reviewing their important contributions to the

maintenance of homeostasis.

A

B

Figure 10–12. Mechanisms
of hormone action. (A) Two-
messenger mechanism of the
action of protein hormones.
(B) Action of steroid hormones.
See text for description.
QUESTION:What must a cell
have in order to be a target cell
for a particular hormone?