556
Chapter 15
slightly farther down the small intestine so that peristaltic
activity slowly migrates down the small intestine, taking about
two hours to reach the large intestine. By the time the MMC
reaches the end of the ileum, new waves are beginning in the
stomach, and the process repeats.
The MMC moves any undigested material still remain-
ing in the small intestine into the large intestine and also
prevents bacteria from remaining in the small intestine long
enough to grow and multiply excessively. In diseases char-
acterized by an aberrant MMC, bacterial overgrowth in the
small intestine can become a major problem. Upon the arrival
of a meal in the stomach, the MMC rapidly ceases in the intes-
tine and is replaced by segmentation.
A rise in the plasma concentration of the intestinal hor-
mone
motilin
is thought to initiate the MMC. Feeding inhib-
its the release of motilin; motilin stimulates MMCs via both
the enteric and autonomic nervous systems.
The contractile activity in various regions of the small
intestine can be altered by refl exes initiated at different points
along the gastrointestinal tract. For example, segmentation
intensity in the ileum increases during periods of gastric emp-
tying, known as the
gastroileal refl
ex.
Large distensions of
the intestine, injury to the intestinal wall, and various bacte-
rial infections in the intestine lead to a complete cessation of
motility, the
intestino-intestinal refl
ex.
As much as 500 ml of air may be swallowed during a
meal. Most of this air travels no farther than the esophagus,
from which it is eventually expelled by belching. Some of the
air reaches the stomach, however, and is passed on to the intes-
tines, where its percolation through the chyme as the intestinal
contents mix produces gurgling sounds that can be quite loud.
Large Intestine
The large intestine is a tube 6.5 cm (2.5 inches) in diameter
and about 1.5 m (5 feet) long. Its fi rst portion, the
cecum,
forms a blind-ended pouch from which extends the
appendix,
a small, fi ngerlike projection that may participate in immune
function but is not essential (
Figure 15–33
). The
colon
consists of three relatively straight segments—the ascending,
transverse, and descending portions. The terminal portion of
the descending colon is S-shaped, forming the sigmoid colon,
which empties into a relatively straight segment of the large
intestine, the rectum, which ends at the anus.
Although the large intestine has a greater diameter than
the small intestine, its epithelial surface area is far smaller,
because the large intestine is much shorter than the small intes-
tine, its surface is not convoluted, and its mucosa lacks villi.
The secretions of the large intestine are scanty, lack digestive
enzymes, and consist mostly of mucus and fl
uid containing
bicarbonate and potassium ions. The primary function of the
large intestine is to store and concentrate fecal material before
defecation.
Chyme enters the cecum through the
ileocecal sphinc-
ter.
This sphincter is normally closed, but after a meal, when
the gastroileal refl ex increases ileal contractions, it relaxes
each time the terminal portion of the ileum contracts, allow-
ing chyme to enter the large intestine. Distension of the large
intestine, on the other hand, produces a refl ex contraction of
the sphincter, preventing fecal material from moving back into
the small intestine.
About 1500 ml of chyme enters the large intestine from
the small intestine each day. This material is derived largely
from the secretions of the lower small intestine because most
of the ingested food is absorbed before reaching the large
intestine. Fluid absorption by the large intestine normally
accounts for only a small fraction of the fl uid absorbed by the
gastrointestinal tract each day.
The primary absorptive process in the large intestine is
the active transport of sodium from lumen to blood, with the
accompanying osmotic absorption of water. If fecal material
remains in the large intestine for a long time, almost all the
water is absorbed, leaving behind hard fecal pellets. There is
normally a net movement of potassium from blood into the
large intestine lumen, probably due to an active mechanism
stimulated by cAMP. Severe depletion of total-body potas-
sium can result when large volumes of fl uid are excreted in the
feces. There is also a net movement of bicarbonate ions into
the lumen coupled to chloride absorption from the lumen,
and loss of this bicarbonate (a base) in patients with prolonged
diarrhea can cause a metabolic acidosis (Chapter 14).
The large intestine also absorbs some of the products
formed by the bacteria colonizing this region. Undigested poly-
saccharides (fi ber) are metabolized to short-chain fatty acids by
bacteria in the large intestine and absorbed by passive diffusion.
The bicarbonate the large intestine secretes helps to neutralize
the increased acidity resulting from the formation of these fatty
acids. These bacteria also produce small amounts of vitamins,
especially vitamin K, for absorption into the blood. Although
this source of vitamins generally provides only a small part of
the normal daily requirement, it may make a signifi cant contri-
bution when dietary vitamin intake is low. An individual who
depends on absorption of vitamins formed by bacteria in the
large intestine may become vitamin defi cient if treated with
Ascending
colon
Cecum
Appendix
Ileum
Transverse
colon
Descending
colon
Sigmoid
colon
Rectum
Figure 15–33
The large intestine begins with the cecum.
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