Major hormones that control digestion
Table 14.1
Hormone
Where produced
Stimulant
Action
Gastrin
Stomach mucosa
(pyloric region)
Stretching of stomach, partially
digested proteins and caffeine in
stomach, and low pH of stomach
chyme.
Stimulates secretion of gastric juice, increases
m otility of GI tract, and relaxes pyloric sphincter.
Secretin
Intestinal mucosa
Acidic chyme that enters the
small intestine.
Stimulates secretion of pancreatic juice rich in
bicarbonate ions.
Cholecystokinin (CCK)
Intestinal mucosa
Amino acids and fatty acids in
chyme in small intestine.
Inhibits gastric emptying, stimulates secretion of
pancreatic juice rich in digestive enzymes, causes
ejection of bile from the gallbladder, and induces a
feeling of satiety (feeling full to satisfaction).
When chyme reaches the ileum, most of the bile salts
are reabsorbed and returned by the blood to the liver for
recycling. Insufficient bile salts, whether caused by ob-
struction of bile ducts or by liver disease, can result in the
loss of up to 40% of dietary lipids in feces due to dimin-
ished lipid absorption.
In sum, these are the functions of the small intestine:
Segmentations mix chyme with digestive juices and
bring food into contact with the mucosa for absorption.
Peristalsis propels chyme through the small intestine.
The
small
intestine
completes
the
digestion
of
carbohydrates, proteins, and lipids and begins and
completes the digestion of nucleic acids.
The small intestine absorbs about 90% of nutrients
and water that pass through the digestive system.
Digestion also involves the activity of a number of
hormones (Table 14.1). Many of these substances are
released into the GI tract and are thought to act locally,
while others are released into the bloodstream. The exact
physiological roles of these “gut hormones” are not known
but are being investigated.
Many of the absorbed nutrients are processed by
the liver. The hepatic portal vein transports nutrient-
rich venous blood to the liver, where it is distributed be-
tween rows of hepatocytes (liver cells) through hepatic
sinusoids. The hepatocytes extract nutrients and process
them. As you will see later in this chapter, hepatocytes
combine simple carbohydrates to form glycogen, make
lipids from glucose and/or amino acids, distribute amino
acids to the blood, and break down excessive amino acids
and worn-out proteins into glucose or triglycerides. Af-
ter extraction and processing, the blood drains into the
central veins to the hepatic vein and ultimately into the
inferior vena cava.
Meanwhile, the remainder of your meal completes its
journey through the small intestine. Segmentation stops
and the peristaltic wave pushes the chyme forward slowly
down the small intestine, reaching the end of the ileum in
90 to 120 minutes. Then another wave of peristalsis begins
in the stomach. Altogether, chyme remains in the small
intestine for 3 to 5 hours. By the time the chyme reaches
the ileum, most nutrients have been absorbed. But water
and some vitamins, carbohydrates, and ions remain. The
final steps of digestion occur in the large intestine. Let’s
turn our attention next to this last organ in the gastroin-
testinal tract.
The Large Intestine Absorbs
Water and Eliminates Wastes
The large intestine is the last part of the GI tract that
your breakfast visits on its travels through the diges-
tive system. It is about 6.5 cm (2.5 in) in diameter and
1.5 m (5 ft) long. It extends from the ileum to the anus
and is attached to the posterior abdominal wall by a
double layer of peritoneum called the mesocolon. It has
four major segments: cecum, colon, rectum, and anal
canal (Figure 14.9). The ileocecal valve regulates the
movement of material into the large intestine, while
the internal and external anal sphincters regulate the
movement of material out of the large intestine.
Like the small intestine, the large intestine needs
lots of surface area to complete its work, which includes
absorption of water, certain ions (Na+ and Cl-), and some
vitamins (B and K). Unlike the small intestine, however,
the large intestine’s surface changes are visible from the
outside of the tubing. Teniae coli, bands of muscle fi-
bers, pucker up the large intestine to form pouch-like
sacs called haustra.
412 CHAPTER 14
The Digestive System, Nutrition, and Metabolism
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