The final portion of the gastrointestinal tract - the large intestine – has 3 major functions:
absorption of water and electrolytes; absorption of vitamins produced by colonic
bacteria; and formation and excretion of feces. The large intestine consists of the cecum, colon,
rectum, and anal canal. The cecum receives food materials from the ileum via the ileocecal valve.
At the lower end of the cecum is the appendix, a winding blind tube containing lymphoid tissue and
a high concentration of intestinal bacteria. The colon starts from the ileocecal junction
and consists of 4 segments: ascending, transverse, descending, and sigmoid colon.
The wall of the large intestine is composed of the same 4 layers as the small intestine,
but there are major differences: - Firstly, there are no circular
folds or villi, only intestinal crypts. - Secondly, the longitudinal muscle layer does not
cover the whole diameter of the colon but exists as 3 ribbon-like bands that run along its length.
Tonic contractions of these bands bunch up the colon into pouches called haustra,
giving it a segmented appearance. The major product of the intestinal
crypts is mucus, although there are endocrine cells producing hormones.
The large intestine does not secrete its own digestive enzymes. Instead, chemical digestion
is carried out by its bacterial flora. These bacteria ferment cellulose and other undigested
carbohydrates, producing several vitamins which are absorbed by the colon. Production of vitamin
K is especially important because a typical diet does not usually meet the body’s requirement for
vitamin K. Bacterial fermentation also releases gases which are expelled out of the body.
During the long passage through the large intestine,
food residues are slowly turned into feces. In addition to undigested food materials,
feces contain significant amounts of bacteria and dead epithelial cells.
There are several types of motility in the colon: - Most common are haustral contractions, in which
a haustrum distends, contracts and pushes its content into the next haustrum. These movements
occur about every 30 min, last for about 1 min, and help promote mixing and absorption.
- Anti-peristaltic contractions, which move food residues back toward the ileocecal valve,
thus slowing down their passage through the colon. - Then there are colonic mass movements, which are
strong waves occurring from mid-transverse colon to the sigmoid colon. They can happen up to four
times a day, often during or immediately after meals, and last for up to 30 minutes.
Mass movements are triggered by the gastrocolic reflex, in which distension of the stomach
increases colon motility. These movements fill the rectum with feces, leading to an urge to defecate.
When colonic motility is slower than usual, or if defecation is delayed for an extended period
of time, the large intestine absorbs more water and constipation may result. On the other hand,
a too fast movement through the colon may cause less water to be absorbed and diarrhea may result.
When the rectum is full, stretch receptors in the rectal wall send nerve impulses to
the sacral region of the spinal cord. By way of a parasympathetic response,
signals return to promote rectal peristalsis and relax the internal anal sphincter. This reflex is
involuntary. However, defecation can only happen when the external anal sphincter, which is under
voluntary control, also opens. This allows delaying defecation until an appropriate time.