The Digestive System
Acute inﬂammation of the appendix, or
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-
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.
has several distinct regions. Proximally, as the
, it travels up the right side of the abdominal cav-
ity to the level of the right kidney. Here it makes a right-angle
right colic (hepatic) ﬂexure
—and travels across the
abdominal cavity as the
. Directly anterior to
the spleen, it bends acutely at the
lef colic (splenic) ﬂexure
descends down the leF side of the posterior abdominal wall as
. Inferiorly, it enters the pelvis, where it
becomes the S-shaped
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
(Figure 23.30c, d)
In the pelvis, at the level of the third sacral vertebra, the
sigmoid colon joins the
, 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
Despite its name (
straight), the rectum has three
lateral curves or bends, represented internally as three trans-
verse folds called
(±igure 23.29b). Tese valves
stop feces from being passed along with gas (ﬂatus).
, 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
. 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
posed of skeletal muscle. Te sphincters, which act rather like
purse strings to open and close the anus, are ordinarily closed
except during defecation.
Te wall of the large intestine diﬀers in several ways from that
of the small intestine. Te large intestine
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
Distension of the stomach and duodenal walls have different
effects on stomach secretory activity. What are these effects?
Which is more important in moving food along the small
intestine—peristalsis or segmentation?
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.
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), 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
sēz). It also
absorbs metabolites produced by resident bacteria as they av-
idly ferment carbohydrates not absorbed in the small intestine.
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
li; “ribbons of the colon”). Teir tone puckers
the wall of the large intestine into pocketlike sacs called
strah; “to draw up”; singular:
). Another obvious
feature of the large intestine is its
ik; “membrane covered”), which are small fat-ﬁlled pouches
of visceral peritoneum that hang from the surface of the large
. 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
“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
. 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.