Human Physiology, 14th edition (2016)

Endocrine Glands 349

Gastrointestinal Tract

The stomach and small intestine secrete a number of hor-
mones that act on the gastrointestinal tract itself and on the
pancreas and gallbladder (chapter 18; the hormone actions are
summarized in table 18.5). These hormones, acting in concert
with regulation by the autonomic nervous system, coordinate
the activities of different regions of the digestive tract and
the secretions of pancreatic juice and bile. Several hormones
secreted by the stomach and small intestine are also known to
stimulate insulin secretion from the pancreatic islets in antici-
pation of a rise in blood glucose following a meal.

Gonads and Placenta

The gonads ( testes and ovaries ) secrete sex steroids. These
include male sex hormones, or androgens, and female sex
hormones— estrogens and progesterone. (Progesterone is pro-
duced by several endocrine glands as a precursor of their hor-
mones, as illustrated in figure 11.2 , but is secreted into the blood
as a major hormone only by the ovaries and placenta.) The
androgens and estrogens are families of hormones. The principal
androgen secreted by the testes is testosterone, and the principal
estrogen secreted by the ovaries is estradiol-17 b. The princi-
pal estrogen during pregnancy, however, is a weaker estrogen
called estriol, secreted by the placenta. After menopause, the
principal estrogen is estrone, produced primarily by fat cells.
The testes consist of two compartments: seminiferous
tubules, which produce sperm cells, and interstitial tissue
between the convolutions of the tubules. Within the interstitial
tissue are Leydig cells, which secrete testosterone (chapter 20;
see fig. 20.11). Testosterone is needed for the development and
maintenance of the male genitalia (penis and scrotum) and the
male accessory sex organs (prostate, seminal vesicles, epidid-
ymides, and vas deferens), as well as for the development of
male secondary sex characteristics.
During the first half of the menstrual cycle, estradiol-17 b
is secreted by small structures within the ovary called ovar-
ian follicles. These follicles contain the egg cell, or ovum, and

granulosa cells that secrete estrogen (chapter 20; see fig. 20.26).

By about midcycle, one of these follicles grows very large and,

in the process of ovulation, extrudes its ovum from the ovary.

The empty follicle, under the influence of luteinizing hormone

(LH) from the anterior pituitary, then becomes a new endocrine

structure called a corpus luteum (see fig. 20.34). The corpus

luteum secretes progesterone as well as estradiol-17 b.

The placenta —the organ responsible for nutrient and waste

exchange between the fetus and mother—is also an endocrine

gland that secretes large amounts of estrogens and progesterone.

In addition, it secretes a number of polypeptide and protein hor-

mones that are similar to some hormones secreted by the anterior

pituitary. These hormones include human chorionic gonadotro-

pin (hCG), which is similar to LH, and somatomammotropin,

which is similar in action to both growth hormone and prolactin.

The physiology of the placenta and other aspects of reproductive

endocrinology are considered in chapter 20.

CLINICAL APPLICATION

Melatonin pills decrease the time required to fall asleep

and increase the duration of rapid eye movement (REM)

sleep; for these reasons, they may be useful in the treat-

ment of insomnia. This is particularly significant for elderly

people with insomnia, who have the lowest nighttime levels

of endogenous melatonin secretion. Melatonin pills can be

used to alleviate jet lag, a sleep disorder produced by cross-

ing time zones too rapidly for the circadian clock in the SCN

to adjust. Endogenous melatonin secretion at night signals

darkness, and so a melatonin pill taken upon arrival at night

before endogenous secretion begins resets the clock to

an earlier time. Taking a melatonin pill at bedtime for a few

nights can therefore be useful after an eastward flight.

| CHECKPOINT

12a. Describe the structure of the endocrine pancreas.

Which cells secrete insulin and which secrete

glucagon?

12b. Describe how insulin and glucagon secretion are

affected by eating and by fasting and explain the

actions of these two hormones.

13. Describe the location of the pineal gland and discuss
    the possible functions of melatonin.


14. Explain how the gonadal and placental hormones are
    categorized and list the hormones secreted by each
    gland.

11.7 Paracrine and Autocrine Regulation

Many regulatory molecules produced throughout the body

act within the organs that produce them. These molecules

may regulate different cells within one tissue, or they may

be produced within one tissue and regulate a different tis-

sue within the same organ.

LEARNING OUTCOMES

After studying this section, you should be able to:

15. Distinguish between autocrine, paracrine, and
    endocrine regulation, and give examples of paracrine
    regulation of blood vessels.


16. Describe the production and significance of the
    prostaglandins.