Chapter 23
The Digestive System
891
23
Defecation
Te rectum is usually empty, but when mass movements force
feces into it, stretching of the rectal wall initiates the
defecation
reflex
. Tis parasympathetic reflex, mediated by the spinal cord,
causes the sigmoid colon and the rectum to contract, and the in-
ternal anal sphincter to relax (
Figure 23.31
1
and
2
). As feces
are forced into the anal canal, messages reach the brain allow-
ing us to decide whether the external (voluntary) anal sphincter
should open or remain constricted to stop passage of feces tem-
porarily (Figure 23.31
3
).
If defecation is delayed, the reflex contractions end within a
few seconds and the rectal walls relax. Te next mass movement
initiates the defecation reflex again—and so on, until the person
chooses to defecate or the urge becomes irresistible.
During defecation, the muscles of the rectum contract to ex-
pel the feces. We aid this process voluntarily by closing the glot-
tis and contracting our diaphragm and abdominal wall muscles
to increase the intra-abdominal pressure (a procedure called
Valsalva’s maneuver
). We also contract the levator ani muscle
Tis condition, called
diverticulosis
, most commonly occurs
in the sigmoid colon, and affects over half of people over age 70.
In about 20% of cases, diverticulosis progresses to
diverticulitis
, in
which the diverticula become inflamed and may rupture, leaking
into the peritoneal cavity, which can be life threatening. Foods and
products that make feces bulkier help prevent diverticulitis.
Irritable bowel syndrome
(
IBS
) is a functional GI disorder not
explained by anatomical or biochemical abnormalities. Affected
individuals have recurring (or persistent) abdominal pain that
is relieved by defecation, changes in the consistency (watery to
stonelike) and frequency of their stools, and varying complaints
of bloating, flatulence, nausea, and depression. Stress is a com-
mon precipitating factor, and stress management is an impor-
tant aspect of treatment.
Te semisolid feces delivered to the rectum, also called the
stool, contain undigested food residues, mucus, sloughed-off
epithelial cells, millions of bacteria, and just enough water to
allow their smooth passage. Of the 500 ml or so of food residue
entering the cecum daily, approximately 150 ml becomes feces.
3
Impulses from
cerebral cortex
(conscious
control)
Voluntary motor
nerve to external
anal sphincter
External anal
sphincter
(skeletal muscle)
Internal anal sphincter
(smooth muscle)
Sensory
nerve fibers
Involuntary motor nerve
(parasympathetic division)
Stretch receptors in wall
Rectum
Sigmoid colon
1
2
Feces move into and
distend the rectum,
stimulating stretch
receptors there. The
receptors transmit signals
along afferent fibers to
spinal cord neurons.
A spinal reflex is initiated in which
parasympathetic motor (efferent) fibers
stimulate contraction of the rectum and
sigmoid colon, and relaxation of the
internal anal sphincter.
If it is convenient to defecate,
voluntary motor neurons are
inhibited, allowing the external
anal sphincter to relax so feces
may pass.
Figure 23.31
Defecation reflex.
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