683
CHAPTER SEVENTEEN
Digestive System
rides sucrose, maltose, and lactose into the monosaccharides
glucose, fructose, and galactose; and
intestinal lipase,
which
splits fats into fatty acids and glycerol.
Table 17.9
summarizes
the sources and actions of the major digestive enzymes.
Regulation of Small Intestinal Secretions
Mucus protects the intestinal wall in the same way it protects
the stomach lining, it is not surprising that mucous secre-
tion increases in response to mechanical stimulation and the
presence of irritants, such as gastric juice. Stomach contents
entering the small intestine stimulate the duodenal mucous
glands to release mucus.
Direct contact with chyme chemically and mechanically
stimulates goblet cells and intestinal glands to secrete their
products. Distension of the intestinal wall activates the nerve
plexuses therein and stimulates parasympathetic reflexes
that also trigger release of small intestine secretions.
PRACTICE
44
Describe the parts of the small intestine.
45
What is the function of an intestinal villus?
46
Distinguish between intestinal villi and microvilli.
47
How is surface area maximized in the small intestine?
48
What is the function of the intestinal glands?
49
List intestinal digestive enzymes.
Villi
Muscular
layer
Submucosa
Plicae
circulares
Circular muscle
Longitudinal muscle
Serosa
(a)
(b)
FIGURE 17.38
Section of small intestine. (
a
) The inner lining of the
small intestine contains many circular folds, the plicae circulares.
(
b
) A longitudinal section through some of these folds.
TABLE
17.9
|
Summary of the Major Digestive Enzymes
Enzyme
Source
Digestive Action
Salivary Enzyme
Salivary amylase
Salivary glands
Begins carbohydrate digestion by breaking down starch and glycogen to disaccharides
Gastric Enzymes
Pepsin
Gastric glands
Begins protein digestion
Gastric lipase
Gastric glands
Begins butterfat digestion
Pancreatic Enzymes
Pancreatic amylase
Pancreas
Breaks down starch and glycogen into disaccharides
Pancreatic lipase
Pancreas
Breaks down fats into fatty acids and glycerol
Trypsin, chymotrypsin
Pancreas
Breaks down proteins or partially digested proteins into peptides
Carboxypeptidase
Pancreas
Breaks down peptides into amino acids
Nucleases
Pancreas
Breaks down nucleic acids into nucleotides
Intestinal Enzymes
Peptidase
Mucosal cells
Breaks down peptides into amino acids
Sucrase, maltase, lactase
Mucosal cells
Breaks down disaccharides into monosaccharides
Intestinal lipase
Mucosal cells
Breaks down fats into fatty acids and glycerol
Enterokinase
Mucosal cells
Shortens trypsinogen into trypsin
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