Digestive System—Physiology of the Small Intestine,
Colon, and Rectum
Lecture 19
We’re going to conclude our look at the gastrointestinal tract now by
reviewing some of the physiology of the small intestine and the colon,
as well as the rectum.
I
n this concluding lecture on the GI tract, we rst examine mechanical
and chemical digestion in the small intestine—the process by which the
small-bowel segments crush and mix the chyme to facilitate absorption
of proteins, lipids, carbohydrates, and water. We turn next to the large
intestine or colon, examining the various re exes that move feces into and
through the colon for excretion through the rectum. Finally, we examine the
physiology of defecation.
The greater omentum hangs from the bottom of the stomach and is composed
mostly of fat. It stores fat and provides a rich blood supply to the stomach.
The lesser omentum is an attachment of the peritoneum that lies between the
liver and the upper edge of the stomach. It carries the vessels that run to the
stomach and liver.
The entire small bowel (duodenum, jejunum, and ileum) is devoted to two
processes: digestion and absorption. Digestion is divided into mechanical
and chemical phases.
Mechanical digestion
Circular muscles constrict and divide the small bowel into segments.
A muscle then contracts between two other muscles and subdivides the
segment. Relaxation allows the segments to coalesce. This is repeated many
times per minute so that the chyme is moved back and forth in the same area.
Localized contractions crush and mix food within that segment alone. This
action mixes the chyme with intestinal juices and prolongs its contact with
the absorptive surface of the small bowel.
111
, Peristaltic contractions, coordinated by the myenteric plexus, move the
chyme slowly along the length of the small bowel. Peristaltic activity is
weak, which means that food stays in the small bowel for a relatively long
time (4–6 hours). Peristalsis is completely under the control of the autonomic
nervous system. Appendicitis and surgery can slow or stop peristalsis.
Because “the solution to pollution is dilution,” the body increases peristalsis
and pours uid into the intestines to eject and weaken toxins in cases such as
cholera. Victims then die of dehydration from massive diarrhea.
Lecture 19: Digestive System—Physiology of the Small Intestine, Colon, and Rectum
Chemical digestion
Chyme in the small bowel is a conglomerate of partially digested
carbohydrates, lipids, and proteins. Digestion must be completed in the small
bowel, because the colon will not absorb nutrients to any signi cant extent.
Crohn’s disease can require the surgical removal of segments of intestine,
sometimes leaving the patient with inadequate bowel to absorb nutrients, or
short bowel syndrome. Speci c chemical digestions include the following:
Proteins
Polypeptides (short-chain amino acids) arrive from protein
breakdown by pepsin in the stomach.
Breakdown continues in the small bowel by activated pancreatic
enzymes: trypsin, chymotrypsin, and elastase.
These three are all necessary because they each act at different
places in the amino acid sequences.
Brush border cells of the small bowel excrete more peptidases—
aminopeptidase and dipeptidase—that complete the splitting of
the amino acids.
Lipids (fats)
Triglycerides (three fatty acids bound to glycerine) are split by
pancreatic lipase in the small bowel.
112
Colon, and Rectum
Lecture 19
We’re going to conclude our look at the gastrointestinal tract now by
reviewing some of the physiology of the small intestine and the colon,
as well as the rectum.
I
n this concluding lecture on the GI tract, we rst examine mechanical
and chemical digestion in the small intestine—the process by which the
small-bowel segments crush and mix the chyme to facilitate absorption
of proteins, lipids, carbohydrates, and water. We turn next to the large
intestine or colon, examining the various re exes that move feces into and
through the colon for excretion through the rectum. Finally, we examine the
physiology of defecation.
The greater omentum hangs from the bottom of the stomach and is composed
mostly of fat. It stores fat and provides a rich blood supply to the stomach.
The lesser omentum is an attachment of the peritoneum that lies between the
liver and the upper edge of the stomach. It carries the vessels that run to the
stomach and liver.
The entire small bowel (duodenum, jejunum, and ileum) is devoted to two
processes: digestion and absorption. Digestion is divided into mechanical
and chemical phases.
Mechanical digestion
Circular muscles constrict and divide the small bowel into segments.
A muscle then contracts between two other muscles and subdivides the
segment. Relaxation allows the segments to coalesce. This is repeated many
times per minute so that the chyme is moved back and forth in the same area.
Localized contractions crush and mix food within that segment alone. This
action mixes the chyme with intestinal juices and prolongs its contact with
the absorptive surface of the small bowel.
111
, Peristaltic contractions, coordinated by the myenteric plexus, move the
chyme slowly along the length of the small bowel. Peristaltic activity is
weak, which means that food stays in the small bowel for a relatively long
time (4–6 hours). Peristalsis is completely under the control of the autonomic
nervous system. Appendicitis and surgery can slow or stop peristalsis.
Because “the solution to pollution is dilution,” the body increases peristalsis
and pours uid into the intestines to eject and weaken toxins in cases such as
cholera. Victims then die of dehydration from massive diarrhea.
Lecture 19: Digestive System—Physiology of the Small Intestine, Colon, and Rectum
Chemical digestion
Chyme in the small bowel is a conglomerate of partially digested
carbohydrates, lipids, and proteins. Digestion must be completed in the small
bowel, because the colon will not absorb nutrients to any signi cant extent.
Crohn’s disease can require the surgical removal of segments of intestine,
sometimes leaving the patient with inadequate bowel to absorb nutrients, or
short bowel syndrome. Speci c chemical digestions include the following:
Proteins
Polypeptides (short-chain amino acids) arrive from protein
breakdown by pepsin in the stomach.
Breakdown continues in the small bowel by activated pancreatic
enzymes: trypsin, chymotrypsin, and elastase.
These three are all necessary because they each act at different
places in the amino acid sequences.
Brush border cells of the small bowel excrete more peptidases—
aminopeptidase and dipeptidase—that complete the splitting of
the amino acids.
Lipids (fats)
Triglycerides (three fatty acids bound to glycerine) are split by
pancreatic lipase in the small bowel.
112
