532
Chapter 15
in the
gastric
(adjective for “stomach”) lumen alters the ion-
ization of polar molecules, especially proteins, disrupting the
extracellular network of connective tissue proteins that form
the structural framework of the tissues in food. The proteins
and polysaccharides released by hydrochloric acid’s dissolv-
ing action are partially digested in the stomach by pepsin and
amylase, the latter contributed by the salivary glands. Fat is a
major food component that is not dissolved by acid.
Hydrochloric acid also kills most of the bacteria that
enter along with food. This process is not completely effective,
and some bacteria survive to colonize and multiply in the gas-
trointestinal tract, particularly the large intestine.
The digestive actions of the stomach reduce food par-
ticles to a solution known as
chyme,
which contains molecu-
lar fragments of proteins and polysaccharides, droplets of fat,
and salt, water, and various other small molecules ingested in
the food. Virtually none of these molecules, except water, can
cross the epithelium of the gastric wall, and thus little absorp-
tion of organic nutrients occurs in the stomach.
Most absorption and the fi nal stages of digestion occur in
the next section of the tract, the
small intestine,
a tube about
2.4 cm in diameter and 3 m in length, that leads from the stom-
ach to the large intestine. (The small intestine is almost twice as
long if removed from the abdomen because the muscular wall
loses its tone.) Hydrolytic enzymes in the small intestine break
down molecules of intact or partially digested carbohydrates,
fats, and proteins into monosaccharides, fatty acids, and amino
acids. Some of these enzymes are on the luminal surface of the
intestinal lining cells, whereas others are secreted by the pan-
creas and enter the intestinal lumen. The products of digestion
are absorbed across the epithelial cells and enter the blood and/
or lymph. Vitamins, minerals, and water, which do not require
enzymatic digestion, are also absorbed in the small intestine.
The small intestine is divided into three segments: an
initial short segment, the
duodenum,
is followed by the
jeju-
num
and then by the longest segment, the
ileum.
Normally,
most of the chyme entering from the stomach is digested and
absorbed in the fi rst quarter of the small intestine, in the duo-
denum and jejunum.
Two major organs—the pancreas and liver—secrete sub-
stances that fl ow via ducts into the duodenum. The
pancreas,
an elongated gland located behind the stomach, has both
endocrine (Chapter 16) and exocrine functions, but only the
latter are directly involved in gastrointestinal function and are
described in this chapter. The exocrine portion of the pan-
creas secretes digestive enzymes and a fl uid rich in bicarbonate
ions. The high acidity of the chyme coming from the stomach
would inactivate the pancreatic enzymes in the small intestine
if the acid were not neutralized by the bicarbonate ions in the
pancreatic fl
uid.
The
liver,
a large organ located in the upper right por-
tion of the abdomen, has a variety of functions, which are
described in various chapters. This is a convenient place to
provide, in
Table 15–2
, a comprehensive reference list of these
hepatic
(meaning “pertaining to the liver”) functions and the
chapters in which they are described. The breadth of hepatic
function is highlighted by the devastating and often lethal
effects of liver failure. We will be concerned in this chapter
only with the liver’s exocrine functions that are directly related
to the secretion of
bile.
Bile contains bicarbonate ions, cholesterol, phospho-
lipids, bile pigments, a number of organic wastes and—most
important—a group of substances collectively termed
bile salts.
The bicarbonate ions, like those from the pancreas, help neu-
tralize acid from the stomach, whereas the bile salts, as we
shall see, solubilize dietary fat. These fats would otherwise be
insoluble in water, and their solubilization increases the rates
at which they are digested and absorbed.
Bile is secreted by the liver into small ducts that join to
form the common hepatic duct. Between meals, secreted bile
is stored in the
gallbladder,
a small sac underneath the liver
that branches from the common hepatic duct. The gallblad-
der concentrates the organic molecules in bile by absorbing
salts and water. During a meal, the smooth muscles in the
gallbladder wall contract, causing a concentrated bile solution
to be injected into the duodenum via the
common bile duct
(
Figure 15–4
), an extension of the common hepatic duct.
The gallbladder can be surgically removed without impairing
bile secretion by the liver or its fl
ow into the intestinal tract. In
fact, many animals that secrete bile do not have a gallbladder.
In the small intestine, monosaccharides and amino acids
are absorbed by specifi
c transporter-mediated processes in the
plasma membranes of the intestinal epithelial cells, whereas
fatty acids enter these cells by diffusion. Most mineral ions are
actively absorbed by transporters, and water diffuses passively
down osmotic gradients.
The motility of the small intestine, brought about by the
smooth muscles in its walls, (1) mixes the luminal contents
with the various secretions, (2) brings the contents into con-
tact with the epithelial surface where absorption takes place,
and (3) slowly advances the luminal material toward the large
intestine. Because most substances are absorbed in the small
intestine, only small volumes of water, salts, and undigested
material pass on to the
large intestine.
The large intestine
temporarily stores the undigested material (some of which is
metabolized by bacteria) and concentrates it by absorbing salts
and water. Contractions of the
rectum,
the fi nal segment of
the large intestine, and relaxation of associated sphincter mus-
cles expel the feces in a process called
defecation.
The average American adult consumes about 500–800 g
of food and 1200 ml of water per day, but this is only a frac-
Table 15–1
Major Functions of Saliva
1. Moistens and lubricates food
2. Digestion of polysaccharides by amylase
3. Dissolves food
4. Antibacterial actions
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