THE LARGE INTESTINE By the time a meal has spent
several hours in the stomach and another five hours or so in the small
intestine, all that remains to enter the large intestine is a combination of water
and indigestible waste, and it enters through a valve that prevents backflow.
The large intestine, or colon, is about two to three inches in diameter. Upon
entering, material travels upward through the ascending colon along the right
side of the body, then through a sharp curve under the liver on the right and
via the transverse colon across the top of the abdomen just below the
diaphragm. Another sharp curve carries the material to the descending colon
along the left side. The lower end of this part leads into an S-shaped section,
the sigmoid colon.
The colon ends in the rectum. Waste is held in the rectum by
a sphincter muscle until it is discharged through the anal opening. Peristaltic
movements in the colon are normally slower than elsewhere in the digestive
tract. Waste often requires 12 to 24 hours to pass through. The colon has no
digestive function but it does serve a vital purpose in absorbing water into
the blood to maintain the water balance of the body.
Indigestible material,
which enters the colon in a watery mixture, becomes nearly solid, because of
the water absorption, by the time it reaches the lower end of the colon.
Even with the absorption of large quantities of
water, feces still consist of two-thirds water. The remainder is made up of
small amounts of food residue, bacteria, intestinal secretions, and intestinal
cellular re- mains-the reason why feces are produced even during starvation.
Under normal conditions, the amount of feces may vary considerably.
On the
average, about 12 ounces of chyme may enter the colon daily (somewhat more on a
rich vegetable diet), and from this will be derived about 4 ounces of feces.
Intestinal gas, or flatus, is natural. It is a mixture of swallowed air and
gases produced by intestinal bacteria. The bacteria quite normally occupy the
digestive tract; some contribute materially to health by producing vitamins.
While waste products are excreted in the form of semisolid feces, some,
including salts and proteins, are filtered from the blood, along with excess
water, by the kidneys and excreted as urine.
