The Digestive System 657SUMMARY
18.4 Large Intestine 632
A. The large intestine is divided into the cecum, colon, rectum,
and anal canal.
1. The appendix is attached to the inferior medial margin
of the cecum.
2. The colon consists of ascending, transverse, descending,
and sigmoid portions.
3. Bulges in the walls of the large intestine are called
haustra.
B. The commensal bacteria of the intestinal microbiota are most
numerous in the large intestine, where they serve a number of
physiologically important functions required for health.
1. The intestinal microbiota produce some B vitamins and
vitamin K, as well as short chain fatty acids.
2. The commensal bacteria of the intestinal microbiota
help protect the GI tract from damaging inflammatory
responses to pathogenic bacteria.
C. The large intestine absorbs water and electrolytes.
1. Although most of the water that enters the GI tract is
absorbed in the small intestine, 1.5 to 2.0 L pass to the
large intestine each day. The large intestine absorbs
about 90% of this amount.
2. Na^1 is actively absorbed and water follows passively,
in a manner analogous to the reabsorption of NaCl and
water in the renal tubules.
D. Defecation occurs when the anal sphincters relax and con-
traction of other muscles raises the rectal pressure.18.5 Liver, Gallbladder, and Pancreas 637
A. The liver, the largest internal organ, is composed of
functional units called lobules.
1. Liver lobules consist of plates of hepatic cells separated
by capillary sinusoids.
2. Blood flows from the periphery of each lobule, where
the hepatic artery and portal vein empty, through the
sinusoids and out the central vein.
3. Bile flows within the hepatocyte plates, in canaliculi, to
the bile ducts.
4. Substances excreted in the bile can be returned to
the liver in the hepatic portal blood. This is called an
enterohepatic circulation.
5. Bile consists of a pigment called bilirubin, bile salts,
cholesterol, and other molecules.
6. The liver detoxifies the blood by excreting substances in
the bile, by phagocytosis, and by chemical inactivation.
7. The liver modifies the plasma concentrations of
proteins, glucose, triglycerides, and ketone bodies.
B. The gallbladder stores and concentrates the bile. It releases
bile through the cystic duct and common bile duct to the
duodenum.
C. The pancreas is both an exocrine and an endocrine gland.
1. The endocrine portion, known as the islets of
Langerhans, secretes the hormones insulin and
glucagon.18.1 Introduction to the Digestive System 619
A. The digestion of food molecules involves the hydrolysis of
these molecules into their subunits.
- The digestion of food occurs in the lumen of the GI tract
and is catalyzed by specific enzymes. - The digestion products are absorbed through the
intestinal mucosa and enter the blood or lymph.
B. The layers (tunics) of the GI tract are, from the inside
outward, the mucosa, submucosa, muscularis, and serosa. - The mucosa consists of a simple columnar epithelium,
a layer of connective tissue called the lamina propria,
and a thin layer of smooth muscle called the muscularis
mucosae. - The submucosa is composed of connective tissue; the
muscularis consists of layers of smooth muscles; the
serosa is connective tissue covered by the visceral
peritoneum. - The submucosa contains the submucosal plexus, and the
muscularis contains the myenteric plexus of autonomic
nerves.
18.2 From Mouth to Stomach 623
A. Peristaltic waves of contraction push food through the lower
esophageal sphincter into the stomach.
B. The stomach consists of a cardia, fundus, body, and pylorus
(antrum). The pylorus terminates with the pyloric sphincter.
- The lining of the stomach is thrown into folds, or rugae,
and the mucosal surface forms gastric pits that lead into
gastric glands. - The parietal cells of the gastric glands secrete HCl; the
chief cells secrete pepsinogen. - In the acidic environment of gastric juice, pepsinogen
is converted into the active protein-digesting enzyme
called pepsin. - Some digestion of protein occurs in the stomach, but the
most important function of the stomach is the secretion
of intrinsic factor, which is needed for the absorption of
vitamin B 12 in the intestine.
18.3 Small Intestine 628
A. The small intestine is divided into the duodenum, jejunum,
and ileum. The common bile duct and pancreatic duct empty
into the duodenum.
B. Fingerlike extensions of mucosa called villi project into the
lumen, and at the bases of the villi the mucosa forms narrow
pouches called the crypts of Lieberkühn.
- New epithelial cells are formed in the crypts.
- The membrane of intestinal epithelial cells is folded
to form microvilli. This brush border of the mucosa
increases the surface area.
C. Digestive enzymes, called brush border enzymes, are located
in the membranes of the microvilli.
D. The small intestine exhibits two major types of
movements—peristalsis and segmentation.