Secretions of the Large Intestine
Secretion. The mucosa of the large intestine, likethat of the small intestine,
has many crypts of Lieberkühn; however, unlike the small intestine, there are
no villi. The epithelial cells contain almost no enzymes. Instead, they consist
mainly of mucous cells that secrete only mucus. The great preponderance of secretion
in the large intestine is mucus. This
mucus contains moderate amounts of bicarbonate ions secreted by a few
non–mucus-secreting epithelial cells. The rate of secretion of mucus is
regulated principally by direct, tactile stimulation of the epithelial cells
lining the large intestine and by local nervous reflexes to the mucous cells in
the crypts of Lieberkühn.
Stimulation of the pelvic
nerves from the spinal cord, which carry parasympathetic innervation to the distal one half to two thirds of
the large intestine, also can cause marked increase in mucus secretion. This
occurs along with increase in peristaltic motility of the colon.
During extreme parasympathetic stimulation, often caused by
emotional disturbances, so much mucus can occasionally be secreted into the
large intestine that the person has a bowel movement of ropy mucus as often as
every 30 minutes; this mucus often contains little or no fecal material.
Mucus in the large intestine protects the intestinal wall against
excoriation, but in addition, it provides an adherent medium for holding fecal
matter together. Furthermore, it protects the intestinal wall from the great
amount of bacterial activity that takes place inside the feces, and, finally,
the mucus plus the alka-linity of the secretion (pH of 8.0 caused by large
amounts of sodium bicarbonate) provides a barrier to keep acids formed in the
feces from attacking the intestinal wall.
Caused by Excess Secretion of Water and Electrolytes in Response to Irritation.
a segment of thelarge intestine becomes intensely irritated, as occurs when
bacterial infection becomes rampant during enteritis,
the mucosa secretes extra large quantities ofwater and electrolytes in addition
to the normal viscid alkaline mucus. This acts to dilute the irritating factors
and to cause rapid movement of the feces toward the anus. The result is diarrhea, with loss of large quanti-ties
of water and electrolytes. But the diarrhea also washes away irritant factors,
which promotes earlier recovery from the disease than might otherwise occur.