HUMAN BIOLOGY

200 Chapter 11

chyme The pasty stomach
contents formed from the
mixing of food with gastric
juice.

gastric juice The fluid
formed as glands in the
stomach lining release HCl,
mucus, enzymes, gastrin,
bicarbonate, and other
substances.

pepsin Enzyme in gastric
juice that helps digest
proteins.

rugae Folds in the inner
wall (mucosa) of the
stomach.

the stomach: Food storage, Digestion, and more

n The stomach is a complex organ with multiple functions in

processing food.

n Links to Exocrine glands 4.1, Epithelial membrane 4.7

The stomach is a muscular, stretchable sac (Figure 11.7A)

with three functions:

1. It mixes and stores ingested food.


2. It produces secretions that help dissolve and break
   down food particles, especially proteins.


3. By way of a sphincter, it helps control the passage of
   food into the small intestine.
   The surface of the stomach wall
   facing the lumen is lined with glan-
   dular epithelium. Each day, gland
   cells in the lining release about 2 liters
   (roughly 2 quarts) of hydrochloric acid
   (HCl), mucus, and other substances.
   These include pepsinogens, precur-
   sors of digestive enzymes called
   pepsins. Other gland cells secrete
   intrinsic factor, a protein required for
   vitamin B 12 to be absorbed later on, in
   the small intestine. Along with water,
   these substances make up the stom-
   ach’s strongly acidic gastric juice.
   Combined with mixing due to stom-
   ach contractions, the acidity converts
   swallowed boluses into thick, pasty chyme (kime). The acid-
   ity also kills most microbes in food.
   The digestion of proteins starts when the high acidity
   denatures proteins and exposes their peptide bonds. The
   acid also converts pepsinogens to active pepsins, which
   break the bonds, “chopping” the protein into fragments.

Meanwhile, gland cells secrete the hormone gastrin, which

stimulates cells that secrete HCl and pepsinogen.

Usually, mucus and bicarbonate prevent gastric juice

from harming the stomach lining. These protections form

the “gastric mucosal barrier.” When the barrier breaks

down, an ulcer can develop, as Section 11.10 describes.

Waves of peristalsis move food out of the stomach.

These waves mix chyme and build force as they approach

the pyloric sphincter at the stomach’s base (Figure 11.7B).

When a strong contraction arrives, the sphincter closes,

squeezing most of the chyme back. Only a small amount

moves into the small intestine at a given time. In this

way the stomach regulates the rate at which food moves

onward, so that food is not passed along faster than it can

be processed. Depending mainly on the fat content and

acidity of chyme, it can take from 2 to 6 hours for a full

stomach to empty. When the stomach is empty, its walls

crumple into folds called rugae.

Water and alcohol are two of a few substances that begin

to be absorbed across the stomach wall. Liquids imbibed

on an empty stomach pass rapidly to the small intestine,

where absorption continues. Putting food into your stom-

ach slows its emptying. This is why the effects of alcohol

are more gradual when drinking accompanies a meal,

especially one that contains fat.

Fig u r e 11.7 Animated! The stomach’s structure allows it to store and mix food and move it onward. a Structure of the stomach.

B How a peristaltic wave moves down the stomach. (© Cengage Learning)

Esophagus

Pyloric

sphincter

Duodenum

Mucosa

Submucosa

Oblique

muscle

Circular

muscle

Longitudinal

muscle

Gastroesophageal Serosa

sphincter

Muscularis

Stomach

a

Partially

processed food

(chyme) enters

the small intestine.

The longitudinal

layer contracts,

shortening and

expanding the

digestive tract and

making space for the

contents to advance.

The circular layer

of the muscularis

contracts in a wave,

constricting the

digestive tract and

pushing the digestive

contents onward.

1 2 3

Pyloric

sphincter

Chyme

B

11.3

What is the stomach’s role in digestion?

• The stomach receives and stores swallowed food. It also
  produces gastric juice that begins chemical digestion,
  especially of proteins.


• A sphincter helps control the pace at which the stomach
  contents empty into the small intestine.

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