Download Digestive Function of the Large Intestine

Digestive Function of the Large Intestine
The major digestive role of the large intestine involves propulsion-pushing fecal matter toward the anus and then out of the body. Chyme,
which stays in the large intestine for 12 to 24 hours, contains few
nutrients. Enteric bacteria are responsible for a small amount of
digestion. The bacterial flora creates vitamins required for normal
metabolism, such as certain B vitamins and vitamin K. Most of the
remaining water and some electrolytes (especially sodium and chloride)
are recycled.
Structure
Action
Outcome
Lumen
Bacterial degradation of chyme
Breaks down undigested proteins,
components
carbohydrates, and amino acids into
substances that can be absorbed and
detoxified by the liver or eliminated in
feces; synthesizes vitamin K and some B
vitamins
Mucosa
Mucus secretionAbsorption
Lubricates colon; protects mucosaAbsorbs
water, solidifies feces, and helps maintain
water balance in body; absorbed solutes
include ions and certain vitamins
Muscularis
externa
Haustral
Muscle contractions move contents
churningPeristalsisDefecation
between haustrumsContractions of
reflex
circular and longitudinal muscles propel
contents along length of colon; Propels
contents into sigmoid colon and
rectumContractions in sigmoid colon and
rectum eliminate feces
Ileostomy
It may surprise you that the large intestine can be completely removed
without significantly affecting digestive functioning. For example, if colon
cancer necessitates the removal of the large intestine, anileostomy can
be performed, in which the terminal ileum is moved out to the abdominal
wall. A sac is then attached to the abdominal wall to collect eliminated
food residues.
Mechanical Digestion
Mechanical digestion in the large intestine begins when chyme moves
from the ileum into the cecum, an activity regulated by the ileocecal
valve. This sphincter is usually partially closed, allowing slow movement
of chyme into the large intestine. Right after we eat, the gastroileal reflex
escalates peristalsis in the ileum, which forces whatever chyme is in the
ileum into the cecum. The activity of the hormone gastrin also relaxes
the ileocecal valve. When the cecum is distended with chyme,
contractions of the ileocecal sphincter strengthen. Once chyme enters
the cecum, colon movements begin. As food residues pass the ileocecal
valve, they fill the cecum and gather in the ascending colon.
The presence of food residue in the colon stimulates slowmoving haustral contractions (haustral churning). These
sluggish segmentations, primarily in the transverse and descending
colons, occur about every 30 minutes and last about one minute. When
a haustra is distended with chyme, its muscle contracts, pushing the
residue into the next haustra. The movements also mix the food residue,
which helps the large intestine absorb water.
The large intestine also has peristaltic movements, but they are slower
than in more proximal portions of the alimentary canal, at a rate of from
three to 12 contractions per minute. The third type of movement in the
large intestine is called mass movements (mass peristalsis). These
drawn-out, slow-moving, but strong peristaltic waves start around the
middle of the transverse colon and quickly force the contents of the
colon into the rectum. Mass movements usually occur three or four
times per day, either while we eat or immediately afterward. Distension
in the stomach and the breakdown products of digestion in the small
intestine provoke the gastrocolic or duogenocolic reflex, which
increases motility, including mass movements, in the colon. Fiber in the
diet both softens the stool and increases the power of colonic
contractions, optimizing the activities of the colon.
Chemical Digestion
The glands of the large intestine secrete only mucus; they do not secrete
digestive enzymes. So chemical digestion in the large intestine occurs
only through the activity of the bacteria in the lumen of the colon.
Bacteria ferment residual carbohydrates in the chyme and discharge the
hydrogen sulfide, carbon dioxide, and methane gases that help
create flatus (gas) in the colon(flatulence refers to excessive flatus).
Some of these gases, including dimethyl sulfide, have foul odors. Each
day, about 500 mL of flatus is produced in the colon. Much more is
produced when we eat some fiber-rich foods such as beans.