Absorption in the Large Intestine: Formation of
Feces
About 1500 milliliters of chyme normally pass through the ileocecal
valve into the large intestine each day. Most of the water and electrolytes in
this chyme are absorbed in the colon, usually leaving less than 100 milliliters
of fluid to be excreted in the feces. Also, essentially all the ions are
absorbed, leaving only 1 to 5 milliequivalents each of sodium and chloride ions
to be lost in the feces.
Most of the absorption in the large intestine occurs in the
proximal one half of the colon, giving this portion the name absorbing colon, whereas the distal
colon functions principally for feces storage until a propitious time for feces
excretion and is therefore called the storage
colon.
Absorption
and Secretion of Electrolytes and Water. Themucosa of the large intestine, like that of
the small intestine, has a high capability for active absorption of sodium, and
the electrical potential gradient created by absorption of the sodium causes
chloride absorp-tion as well. The tight junctions between the epithelial cells
of the large intestinal epithelium are much tighter than those of the small
intestine. This prevents signif-icant amounts of back-diffusion of ions through
these junctions, thus allowing the large intestinal mucosa to absorb sodium
ions far more completely—that is, against a much higher concentration
gradient—than can occur in the small intestine. This is especially true when large
quantities of aldosterone are available because aldosterone greatly enhances
sodium trans-port capability.
In addition, as occurs in the distal portion of the small
intestine, the mucosa of the large intestine secretes bicarbonate ions while it simultaneously absorbs an equal number of
chloride ions in an exchange transport process that has already been described.
The bicarbonate helps neutralize the acidic end products of bacterial action in
the large intestine.
Absorption of sodium and chloride ions creates an osmotic gradient
across the large intestinal mucosa, which in turn causes absorption of water.
Maximum
Absorption Capacity of the Large Intestine. Thelarge intestine can absorb a maximum of 5 to
8 liters of fluid and electrolytes each day. When the total quan-tity entering
the large intestine through the ileocecal valve or by way of large intestine
secretion exceeds this amount, the excess appears in the feces as diar-rhea. As
noted earlier, toxins from cholera or certain other bacterial infections often
cause the crypts in the terminal ileum and in the large intestine to secrete 10
or more liters of fluid each day, leading to severe and sometimes lethal
diarrhea.
Bacterial Action in the Colon. Numerous bacteria, especiallycolon bacilli, are present even
normally in the absorbing colon. They are capable of digesting small amounts
ofcellulose, in this way providing a few calories of extra nutrition for the
body. In herbivorous animals, this source of energy is significant, although it
is of negligi-ble importance in human beings.
Other substances formed as a result of bacterial activ-ity are
vitamin K, vitamin B12, thiamine, riboflavin, and various gases that
contribute to flatus in the colon,
espe-cially carbon dioxide, hydrogen gas, and methane. The bacteria-formed
vitamin K is especially important because the amount of this vitamin in the
daily ingested foods is normally insufficient to maintain adequate blood
coagulation.
Composition of the Feces. The feces normally are
aboutthree-fourths water and one-fourth
solid matter that itself is composed
of about 30 per cent dead bacteria,
10 to 20 per cent fat, 10 to 20 per
cent inorganic matter, 2 to 3 per
cent protein, and 30 per cent undigestedroughage from the food and
dried constituents of diges-tive juices, such as bile pigment and sloughed
epithelial cells. The brown color of feces is caused by stercobilin and urobilin,
derivatives of bilirubin. The odor is caused principally by products of
bacterial action; these prod-ucts vary from one person to another, depending on
each person’s colonic bacterial flora and on the type of food eaten. The actual
odoriferous products include indole, skatole, mercaptans, and hydrogen
sulfide.
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