The Digestion and Absorption of Food
555
that the intestinal epithelium at the base of the villi secretes a
number of mineral ions, notably sodium, chloride, and bicar-
bonate ions into the lumen, and water follows by osmosis.
These secretions, along with mucus, lubricate the surface of
the intestinal tract and help protect the epithelial cells from
excessive damage by the digestive enzymes in the lumen. Some
damage to these cells still occurs, however, and the intestinal
epithelium has one of the highest cell renewal rates of any tis-
sue in the body.
Chloride is the primary ion determining the magnitude
of fl uid secretion. Various hormonal and paracrine signals—as
well as certain bacterial toxins—can increase the opening fre-
quency of these channels and thus increase fl
uid secretion.
As stated earlier, water movement into the lumen also
occurs when the chyme entering the small intestine from the
stomach is hypertonic because of a high concentration of sol-
utes in the meal, and because digestion breaks down large
molecules into many more small molecules. This hypertonic-
ity causes the osmotic movement of water from the isotonic
plasma into the intestinal lumen.
Absorption
Normally, virtually all of the fl uid secreted by the small intes-
tine is absorbed back into the blood. In addition, a much
larger volume of fl uid, which includes salivary, gastric, hepatic,
and pancreatic secretions, as well as ingested water, is simul-
taneously absorbed from the intestinal lumen into the blood.
Thus, overall there is a large net absorption of water from
the small intestine. Absorption is achieved by the transport
of ions, primarily sodium, from the intestinal lumen into the
blood, with water following by osmosis.
Motility
In contrast to the peristaltic waves that sweep over the stom-
ach, the most common motion in the small intestine during
digestion of a meal is a stationary contraction and relaxation of
intestinal segments, with little apparent net movement toward
the large intestine (
Figure 15–32
). Each contracting segment
is only a few centimeters long, and the contraction lasts a few
seconds. The chyme in the lumen of a contracting segment is
forced both up and down the intestine. This rhythmical con-
traction and relaxation of the intestine, known as
segmen-
tation,
produces a continuous division and subdivision of
the intestinal contents, thoroughly mixing the chyme in the
lumen and bringing it into contact with the intestinal wall.
These segmenting movements are initiated by electrical
activity generated by pacemaker cells in or associated with the
circular smooth muscle layer. As with the slow waves in the
stomach, this intestinal basic electrical rhythm produces oscil-
lations in the smooth muscle membrane potential. If threshold
is reached, action potentials are triggered that increase muscle
contraction. The frequency of segmentation is set by the fre-
quency of the intestinal basic electrical rhythm, but unlike
the stomach, which normally has a single rhythm (three per
minute), the intestinal rhythm varies along the length of the
intestine, each successive region having a slightly lower fre-
quency than the one above. For example, segmentation in the
duodenum occurs at a frequency of about 12 contractions/
min, whereas in the last portion of the ileum the rate is only 9
contractions/min. Segmentation produces, therefore, a slow
migration of the intestinal contents toward the large intestine
because more chyme is forced downward, on the average, than
upward.
The intensity of segmentation can be altered by hormones,
the enteric nervous system, and autonomic nerves; parasympa-
thetic activity increases the force of contraction, and sympathetic
stimulation decreases it. Thus, cephalic phase stimuli, including
emotional states, can alter intestinal motility. As is true for the
stomach, these inputs produce changes in the force of smooth
muscle contraction but do not signifi cantly change the fre-
quencies of the basic electrical rhythms.
After most of a meal has been absorbed, the segment-
ing contractions cease and are replaced by a pattern of peri-
staltic activity known as the
migrating myoelectric complex
(MMC).
Beginning in the lower portion of the stomach,
repeated waves of peristaltic activity travel about 2 feet along
the small intestine and then die out. The next MMC starts
Site of first contraction
Time
Figure 15–32
Segmentation contractions in a portion of the small intestine in
which segments of the intestines contract and relax in a rhythmic
pattern but do
not
undergo peristalsis. This is the rhythm
encountered during a meal. Dotted lines are reference points to
show the site of the fi rst contraction in time (starting at the top). As
contractions occur at the next site, the chyme is divided and pushed
back and forth, mixing the luminal contents.
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