Chapter 23
The Digestive System
887
23
Homeostatic Imbalance
23.12
Acute inflammation of the appendix, or
appendicitis
, results
from a blockage (oFen by feces) that traps infectious bacteria
in its lumen. Unable to empty its contents, the appendix swells,
squeezing off venous drainage, which may lead to ischemia and
necrosis (death and decay) of the appendix. If the appendix rup-
tures, feces containing bacteria spray over the abdominal con-
tents, causing
peritonitis
.
Te symptoms of appendicitis vary, but the first symptom
is usually pain in the umbilical region. Loss of appetite, nausea
and vomiting, and pain relocalization to the lower right abdom-
inal quadrant follow. Immediate surgical removal of the appen-
dix (appendectomy) is the accepted treatment. Appendicitis is
most common during adolescence, when the entrance to the
appendix is at its widest.
Te
colon
has several distinct regions. Proximally, as the
as-
cending colon
, it travels up the right side of the abdominal cav-
ity to the level of the right kidney. Here it makes a right-angle
turn—the
right colic (hepatic) flexure
—and travels across the
abdominal cavity as the
transverse colon
. Directly anterior to
the spleen, it bends acutely at the
lef colic (splenic) flexure
and
descends down the leF side of the posterior abdominal wall as
the
descending colon
. Inferiorly, it enters the pelvis, where it
becomes the S-shaped
sigmoid colon
.
Te colon is retroperitoneal, except for its transverse and sig-
moid parts. Tese parts are intraperitoneal and anchored to the
posterior abdominal wall by mesentery sheets called
mesoco-
lons
(Figure 23.30c, d)
.
In the pelvis, at the level of the third sacral vertebra, the
sigmoid colon joins the
rectum
, which runs posteroinferiorly
just in front of the sacrum. Te position of the rectum allows
a number of pelvic organs (e.g., the prostate of males) to be ex-
amined digitally (with a finger) through the anterior rectal wall.
Tis is called a
rectal exam
.
Despite its name (
rectum
5
straight), the rectum has three
lateral curves or bends, represented internally as three trans-
verse folds called
rectal valves
(±igure 23.29b). Tese valves
stop feces from being passed along with gas (flatus).
Te
anal canal
, the last segment of the large intestine, lies in
the perineum, entirely external to the abdominopelvic cavity.
About 3 cm long, it begins where the rectum penetrates the leva-
tor ani muscle of the pelvic floor and opens to the body exterior
at the
anus
. Te anal canal has two sphincters, an involuntary
internal anal sphincter
composed of smooth muscle (part of
the muscularis), and a voluntary
external anal sphincter
com-
posed of skeletal muscle. Te sphincters, which act rather like
purse strings to open and close the anus, are ordinarily closed
except during defecation.
Microscopic Anatomy
Te wall of the large intestine differs in several ways from that
of the small intestine. Te large intestine
mucosa
is simple co-
lumnar epithelium except in the anal canal. Because most food
is absorbed before reaching the large intestine, there are no cir-
cular folds, no villi, and virtually no cells that secrete digestive
the ileum. Tis reflex sweeps the contents of the previous meal
completely out of the stomach and small intestine as the next
meal is eaten.
Check Your Understanding
41.
Distension of the stomach and duodenal walls have different
effects on stomach secretory activity. What are these effects?
42.
Which is more important in moving food along the small
intestine—peristalsis or segmentation?
43.
What is the MMC and why is it important?
For answers, see Appendix H.
The Large Intestine
List the major functions of the large intestine.
Describe the regulation of defecation.
Te
large intestine
frames the small intestine on three sides and
extends from the ileocecal valve to the anus (see ±igure 23.1).
Its diameter, at about 7 cm, is greater than that of the small
intestine (hence,
large
intestine), but it is much shorter (1.5 m
versus 6 m). Its major digestive functions are to absorb most of
the remaining water from indigestible food residues (delivered
to it in a fluid state), store the residues temporarily, and then
eliminate them from the body as semisolid
Feces
(fe
9
sēz). It also
absorbs metabolites produced by resident bacteria as they av-
idly ferment carbohydrates not absorbed in the small intestine.
Gross Anatomy
Te large intestine exhibits three features not seen elsewhere—
teniae coli, haustra, and epiploic appendages. Except for its
terminal end, the longitudinal muscle layer of its muscularis is
mostly reduced to three bands of smooth muscle called
teniae
coli
(ten
9
ne-e ko
9
li; “ribbons of the colon”). Teir tone puckers
the wall of the large intestine into pocketlike sacs called
haustra
(haw
9
strah; “to draw up”; singular:
haustrum
). Another obvious
feature of the large intestine is its
epiploic appendages
(ep
0
ĭ-
plo
9
ik; “membrane covered”), which are small fat-filled pouches
of visceral peritoneum that hang from the surface of the large
intestine
(Figure 23.29a)
. Teir significance is not known.
Subdivisions of the Large Intestine
Te large intestine has the following subdivisions: cecum, appen-
dix, colon, rectum, and anal canal. Te saclike
cecum
(se
9
kum;
“blind pouch”), which lies below the ileocecal valve in the right
iliac fossa, is the first part of the large intestine (±igure 23.29a).
Attached to the posteromedial surface of the cecum is the
blind, wormlike
appendix
. Te appendix contains masses of lym-
phoid tissue, and as part of MAL² (see p. 759) it plays an impor-
tant role in body immunity. Additionally, it serves as a storehouse
of bacteria and recolonizes the gut when needed. However, the
appendix has an important structural shortcoming—its twisted
structure provides an ideal location for enteric bacteria to accu-
mulate and multiply.
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