amino acids, vitamin C and the B vitamins, miner-
als, and water. Blood from the small intestine goes
to the liver first by way of portal circulation.- The villi contain lacteals (lymph capillaries) for the
absorption of fat-soluble nutrients: vitamins A, D,
E, and K, fatty acids, and glycerol, which are com-
bined to form chylomicrons. Lymph from the small
intestine is carried back to the blood in the left sub-
clavian vein.
Large Intestine (colon)—extends from the
small intestine to the anus
- Colon—parts (see Fig. 16–9): cecum, ascending
colon, transverse colon, descending colon, sigmoid
colon, rectum, anal canal. - Ileocecal valve—at the junction of the cecum and
ileum; prevents backup of fecal material into the
small intestine. - Colon—functions: absorption of water, minerals,
vitamins; elimination of undigestible material. - Normal flora—the bacteria of the colon; produce
vitamins, especially vitamin K, and inhibit the
growth of pathogens. - Defecation reflex—stimulus: stretching of the rec-
tum when peristalsis propels feces into it. Sensory
impulses go to the sacral spinal cord, and motor
impulses return to the smooth muscle of the rec-
tum, which contracts. The internal anal sphincter
relaxes to permit defecation. Voluntary control is
provided by the external anal sphincter, made of
skeletal muscle (see Fig. 16–10).
Liver—other functions- Carbohydrate metabolism—excess glucose is
stored in the form of glycogen and converted back
to glucose during hypoglycemia; fructose and
galactose are changed to glucose. - Amino acid metabolism—the non-essential amino
acids are synthesized by transamination; excess
amino acids are changed to carbohydrates or fats by
deamination; the amino groups are converted to
urea and excreted by the kidneys. - Lipid metabolism—formation of lipoproteins for
transport of fats in the blood; synthesis of choles-
terol; excretion of excess cholesterol into bile; beta-
oxidation of fatty acids to form two-carbon acetyl
groups for energy use. - Synthesis of plasma proteins—albumin to help
maintain blood volume; clotting factors for blood
clotting; alpha and beta globulins as carrier mole-
cules. - Formation of bilirubin—old RBCs are phagocy-
tized, and bilirubin is formed from the heme and
put into bile to be eliminated in feces. - Phagocytosis by Kupffer cells—fixed macrophages;
phagocytize old RBCs and bacteria, especially bac-
teria absorbed by the colon. - Storage—vitamins: B 12 , A, D, E, and K, and the
minerals iron and copper. - Detoxification—liver enzymes change potential
poisons to less harmful substances; examples of
toxic substances are alcohol, medications, and
ammonia absorbed by the colon.
392 The Digestive System
REVIEW QUESTIONS
- Name the organs of the alimentary tube, and
describe the location of each. Name the accessory
digestive organs, and describe the location of each.
(pp. 370, 372, 373, 376, 378, 379, 385) - Explain the purpose of mechanical digestion, and
give two examples. Explain the purpose of chemical
digestion, and give two examples. (pp. 370, 374) - Name the end products of digestion, and explain
how each is absorbed in the small intestine.
(pp. 370, 384) - Explain the function of teeth and tongue, salivary
amylase, enamel of teeth, lysozyme, and water of
saliva. (pp. 370–372)
5. Describe the function of the pharynx, esophagus,
and lower esophageal sphincter. (p. 373)
6. Name and describe the four layers of the alimen-
tary tube. (pp. 373, 376)
7. State the two general functions of the stomach and
the function of the pyloric sphincter. Explain the
function of pepsin, HCl, and mucus. (pp. 376–378)
8. Describe the general functions of the small intes-
tine, and name the three parts. Describe the struc-
tures that increase the surface area of the small
intestine. (pp. 378, 383–384)
9. Explain how the liver, gallbladder, and pancreas
contribute to digestion. (pp. 379, 381)