While
the stomach digests proteins into peptides
through the action of pepsin, the majority of food is di-
gested and absorbed in the small intestine. Let’s take a
closer look at the role of the small intestine in transform-
ing your breakfast into fuel.
The Small Intestine Is the Site of Most
Digestion and Absorption
After the stomach turns your breakfast into chyme and
empties it into the small intestine, the real work of diges-
tion begins. The small intestine consists of three parts—
duodenum, jejunum, and ileum (Figure 14.6)—and is
about 5 m (16 ft) long in an adult.
The small intestine has the
same layers as other portions of
the GI tract, but the inner surface
is highly folded, both macroscopi-
cally into circular folds and micro-
scopically into
villi
and microvilli.
This folded surface is essential
for the functioning of the small
intestine, where the vast major-
ity of materials are digested and
absorbed. These internal features of the small intestine
increase the surface area by 300-400% while maintaining
a relatively small outer surface for the organ. The villi are
lined with absorptive cells, secretory cells, and endocrine
cells. In the crevices between villi are intestinal glands,
which secrete intestinal juices that contain water, mucus,
and enzymes.
Digestion in the intestine starts at the duodenum,
which is the first part of the small intestine and is about
25 cm (10 in) long. The jejunum and ileum are each
about 2-2.5 m (6-8 ft) long. Acidic chyme enters the duo-
denum and stimulates S cells to produce secretin, which
in turn stimulates the pancreas to secrete pancreatic bi-
carbonate into the duodenum (Figure 14.7 on the next
page). The bicarbonate-rich pancreatic juice neutralizes
the acid in the chyme. The presence of amino acids and
fatty acids in the chyme stimulates the CCK cells to se-
crete cholecystokinin (CCK), which increases pancre-
atic enzyme secretion. CCK also causes bile release from
the gallbladder and inhibits gastric emptying. Local-
ized muscle movements called segmentations mix the
various secretions with the chyme. Peristalsis moves the
chyme along the length of the small intestine as diges-
tion and absorption occur.
villi
(VIL-T) Projec-
tions of the intestinal
mucosal cells contain-
ing connective tissue,
blood vessels, and
a lymphatic vessel
that function in the
absorption of the end
products of digestion.
InSight
T h e s m a l l i n t e s t i n e
F ig u r e 1 4 . 6
The small intestine consists of three regions (duo-
denum, jejunum, ileum) with highly folded layers.
Water-soluble substances absorbed through the
epithelial lining go into blood vessels. Fat-soluble
substances proceed into lymph vessels called
lacteals and eventually make their way to the blood
circulation.
Duodenum
-------
1
Jejunum
-----------
Ileum
---------------
Stomach
Large
intestine
Anterior view of external anatomy
Circular folds of mucosa
and submucosa increase
the surface area for
absorption of nutrients.
Internal anatomy of the jejunum
408 CHAPTER 14
The Digestive System, Nutrition, and Metabolism
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