HUMAN BIOLOGY

DiGestioN aND NUtritioN 207

hoW does the body regulate food digestion?

• Signals from nerves and hormones control activity in the
  digestive system.


• Sensory receptors in the mouth and in the wall of the stomach
  and small intestine trigger nervous system controls.

taKe-Home message

n Nerves and hormones regulate food digestion.

Signals from the nervous system and hormones from
endocrine cells jointly regulate digestion. These controls
are sensitive to the presence of food in the GI tract and the
food’s chemical makeup (Figure 11.13).
When you take food in your
mouth—and sometimes when
you merely think about eating—
sensory receptors in your mouth
stimulate the salivary glands to
release saliva. Food entering the
stomach stretches the stomach
walls, and then those of the small intestine. This stretch-
ing also triggers signals from sensory receptors. Some
of the signals give you (by way of processing in your
brain) that “full” feeling after you eat. Others can lead
to the muscle contractions of peristalsis or
the release of digestive enzymes and other
substances. Centers in the brain coordinate
these activities with factors such as how much
blood is flowing to the small intestine, where
nutrients are being absorbed.
There are several types of endocrine cells
in the GI tract (Table 11.2). For example, one
type secretes the hormone gastrin into the
bloodstream when the stomach contains pro-
tein. Gastrin mainly stimulates the release of
hydrochloric acid (HCl), which you may recall
is a key ingredient in gastric juice. After the
stomach has emptied out, the increased acidity
there causes another type of endocrine cell to
release somatostatin, which shuts down HCl
secretion so that conditions in the stomach are
less acidic. Notice that this is an example of
negative feedback.
Hormones also come from endocrine cells
in the small intestine. One of them, secretin,
signals the pancreas to release bicarbonate
when acid enters the duodenum. When fat
enters the small intestine, a hormone called
CCK (for cholecystokinin) is released. CCK
spurs the pancreas to release enzymes and triggers
gallbladder contractions that deliver bile into the small
intestine. Secretin and CCK also slow the rate at which
the stomach empties—the mechanism mentioned in
Section 11.3 that prevents food from entering the small
intestine faster than it can be processed there. Yet another
hormone, GIP (for glucose-dependent insulinotropic pep-
tide) is released when fat and glucose are in the small
intestine. GIP stimulates the release of insulin from the
pancreas, which is required for cells to take up glucose.

Controls over Digestion

F i g u r e 11.13 Hormones and the nervous system act to control

various aspects of digestion. (© Cengage Learning)

Other receptors in the

stomach respond to

food, signaling the

nervous system to

stimulate the stomach

to secrete the hormone

gastrin. It in turn

stimulates the stomach

to secrete HCI and

pepsinogen.

Stretch receptors

in the stomach

respond to food,

signaling the

nervous system to

increase stomach

contractions.

Receptors in the

mouth respond to

food by increasing

secretion of saliva.

Hormone controls Receptor controls

Acidic chyme stimulates

release of the hormone

secretin in the small intestine.

Secretin inhibits motility in the

small intestine and stimulates

HCO 3 – secretion into the

duodenum.

Fat in chyme stimulates

release of the hormone

CCK (cholecystokinin).

CCK inhibits stomach

emptying and stimulates

secretion of pancreatic

enzymes.

Food entering the GI

tract stimulates GIP

secretion, which triggers

release of the hormone

insulin. Insulin stimulates

the uptake of glucose

from blood.

11.8

Hormone Source effects on Digestive System

Gastrin Stomach Increases acid secretion by

stomach

Somatostatin Pancreas Halts HCl secretion in the

empty stomach

Cholecystokinin Small Increases enzyme secretion

(CCK) intestine by pancreas and causes

contraction of gallbladder

Secretin Small Increases bicarbonate

intestine secretion by pancreas and

slows contractions in the

small intestine

GIP Small Stimulates pancreas to

intestine release insulin

Table 11.2 Hormonal Controls of Digestion

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