HUMAN BIOLOGY

300 Chapter 15

biological clock Internal
mechanism by which the
body may monitor day
length.

gonads Primary sex
organs—testes in males
and ovaries in females.

pineal gland Endocrine
gland that produces the
hormone melatonin.

What are some other examples of hormone

sources in the body?

• A female’s ovaries or a male’s testes are gonads that produce
  sex hormones as well as gametes (eggs or sperm).


• The pineal gland is in the brain and produces melatonin, which
  influences sleep–wake cycles and the onset of puberty.


• Skeletal muscle produces irisin, which converts white fat to
  brown fat.


• The thymus is in the chest and secretes thymosins that are
  necessary for the maturation of T cells.


• The heart atria produce ANP, which helps regulate blood
  pressure.


• The GI tract produces hormones that have roles in digestion.

taKe-home message

15.11 Other Hormone Sources

n    Endocrine cells in sex organs, parts of the brain, the

thymus, the heart, and the Gi tract make hormones.

n Links to Heart 7.2, Controls over digestion 11.8,

Lymphatic system 9.2

The gonads produce sex hormones

The human primary sex organs are called gonads. Most

people know them as the ovaries in females and testes

in males (Figure 15.12). In addition to producing sex

cells—eggs in ovaries, sperm in testes—the gonads also

make sex hormones. Ovaries make

estrogens and progesterone. The testes

make mostly testosterone, but they

also make a little estrogen and pro-

gesterone. Small amounts of these

“female” hormones are required for

proper development of sperm. Simi-

larly, a female’s ovaries make small

amounts of testosterone. It contrib-

utes to libido, the desire for sex.

The pineal gland makes melatonin

Many ancient vertebrates had a light-sensitive “third eye”

on top of the head. In humans a version of this organ still

exists, as a lump of tissue in the brain called the pineal

gland. It releases the hormone melatonin into cerebrospinal

fluid and the bloodstream. Melatonin influences sleep/

wake cycles. It is secreted in the dark, so the amount in

the bloodstream varies from day to night. It also changes

with the seasons, because winter days are shorter than

summer days.

The human cycle of sleep and arousal is evidence of an

internal biological clock that apparently monitors day

length. Melatonin seems to influence the clock, which can

be disturbed by circumstances that alter a person’s accus-

tomed exposure to light and dark. Jet lag is an example.

Some air travelers use melatonin supplements to try to

adjust their sleep/wake cycles more quickly.

Depression, intense sleepiness, and other symptoms

of seasonal affective disorder, or SaD, hit some people

in winter. SAD may be due to a biological clock that is

out of sync with changes in day length during winter,

when days are shorter and nights longer. The symptoms

get worse if a person takes melato-

nin. They improve when the person

is ex posed to intense light, which

shuts down the pineal gland.

Melatonin may affect the gonads.

A decline in melatonin production

starts at puberty and may help trig-

ger it. Some pineal gland disorders

accelerate or delay puberty.

skeletal muscle, the thymus, the heart, and

the gi tract also produce hormones

Your Future in Chapter 6 mentioned that a newly discov-

ered hormone, dubbed irisin (after the mythological Greek

goddess Iris who acted as a messenger), is produced in

skeletal muscle during and after exercise. Irisin’s targets

include white adipose cells, which respond by convert-

ing to a form (brown fat) that is more readily “burned” as

bodily fuel. The thymus gland (see Figure 15.1) releases

hormones called thymosins that help infection-fighting

T cells mature. The two heart atria secrete atrial natriuretic

peptide, or ANP. When your blood pressure rises, ANP acts

to inhibit the reabsorption of sodium ions—and hence

water—in the kidneys. More water is excreted, the blood

volume decreases, and blood pressure falls.

Chapter 11 noted that the GI tract produces several hor-

mones that influence appetite or have roles in digestion.

For example, gastrin stimulates the release of stomach acid

when proteins are being digested. Secretin stimulates the

pancreas to secrete bicarbonate.

testis

(where sperm

originate)

ovary

(where eggs

develop)

Figure 15.12 Male and female gonads produce sex hormones

as well as sex cells (sperm in males and eggs in females).

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