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Large Intestine
The large intestine is the terminal part of the gastrointestinal tract.
The primary digestive function of this organ is to finish absorption,
produce some vitamins, form feces, resorb water and eliminate feces
from the body.
The large intestine runs from the cecum, where it attches to the ileum,
to the anus. It borders the small intestine on three sides. Despite its
being around half as long as the small intestine – 4.9 feet versus 10 feet
(1.5 – 3 meters) – it is called the large intestine because it is more than
twice the diameter of the small intestine, 2.5 inches versus one inch (6
cm versus 2.5 cm). The large intestine is tethered to the posterior
abdominal wall by the mesocolon, a double layer of peritoneal
membrane.
The large intestine is subdivided into four main regions: the cecum, the
colon, the rectum, and the anus. The ileocecal valve, located at the
opening between the ileum in the small intestine and the large intestine,
controls the flow of chyme from the small to the large intestine.
Large Intestine Anatomical Structures
Like the small intestine, the mucosa of the large intestine has intestinal
glands that contain both absorptive and goblet cells. However, there are
several notable differences between the walls of the large and small
intestines. For example, other than the anal canal, the mucosa of the
colon is simple columnar epithelium. In addition, the wall of the large
intestine has no circular folds, no villi, and essentially no enzymesecreting cells. This is because most nutrients are already absorbed
before chyme enters the large intestine. The large intestinal wall does
have thicker mucosa and deeper – and more abundant – glands that
contain a vast number of goblet cells. These goblet cells secrete mucus
that eases the movement of feces and protects the intestine from the
effects of the acids and gases produced by enteric bacteria.
Anatomical structures of the large intestine. This work by Cenveo is licensed under a Creative Commons
Attribution 3.0 United States (http://creativecommons.org/licenses/by/3.0/us/).
The stratified squamous epithelial mucosa of the anal canal joins with
the skin around the anus. This mucosa varies considerably from that of
the rest of the colon to accommodate the increased abrasion in this
region. The anal canal's mucous membrane is organized in longitudinal
folds called anal columns that house a grid of veins. Depressions
between the anal columns, called anal sinuses, secrete mucus when
feces crowd them. This facilitates defecation. The pectinate line is a
horizontal, jagged band that runs alongside the inferior margins of the
anal sinuses. The mucosa superior to this line is fairly insensitive to
pain, while the area inferior to this line is very pain-sensitive. The
difference in pain response is due to the fact that the superior region is
innervated by visceral sensory fibers, and the inferior region is
innervated by somatic sensory fibers. There are two superficial venous
plexuses in the anal canal – one with the anus and the other with the
anal columns. Inflammation and distension of these (hemorrhoidal)
veins causes hemorrhoids, an itchy condition caused by the swelling of
these vessels.
Three features are unique to the large intestine: teniae coli, haustra, and
epiploic appendages. The teniae coli are three bands of smooth muscle
that make up the longitudinal muscle layer of the muscularis externa of
the large intestine, except at its terminal end in the rectum. Tonic
contractions of the teniae coli bunch up the colon into a succession of
pouches called haustra, which are responsible for the wrinkled
appearance of the colon. Attached to the teniae coli are small, fat-filled
sacs of visceral peritoneum called epiploic appendages (omental
appendices). These fatty pouches of peritoneum found in the serosa
from the transverse colon through the sigmoid colon.
Although the rectum and anal canal have no teniae coli or haustra, they
do have well-developed layers of muscularis externa muscle that create
the strong contractions needed for defecation.
Large Intestine Gross Anatomy
The first part of the large intestine is the cecum, a small sac-like region
that is suspended inferior to the ileocecal valve. This cecum is about 2.4
inches long. The appendix or vermiform appendix (vermiform=
“worm-shaped”, and appendix = “appendage”) is a winding, coiled tube
that attaches to the cecum. This 2-7 cm (~3 inch) long appendix
contains lymphoid tissue and plays an important role in immunity.
Nevertheless, its twisted anatomy provides a haven for the accumulation
and multiplication of enteric bacteria. The mesoappendix, the
mesentery of the appendix, tethers it to the inferior part of the
mesentery of the ileum.
Gross anatomy of the large intestine. This work by Cenveo is licensed under a Creative Commons Attribution
3.0 United States (http://creativecommons.org/licenses/by/3.0/us/).
The end of the cecum joins with the colon, a long tube with several
distinct areas. The ascending colonruns up the right side of the
abdomen. At the inferior surface of the liver, it takes a right-angle turn,
forming
the right
colic (hepatic)
flexure
and
becoming
the transverse colon. The transverse colon runs across to the left side
of the abdomen. It then bends sharply at a point immediately anterior to
the spleen, forming theleft colic (splenic) flexure. As
the descending colon, it runs down the left side of the posterior
abdominal wall. After entering the pelvis inferiorly, it becomes the sshaped sigmoid colon, which extends medially to the midline. Most of
the colon is between the peritoneal membrane and the body wall, except
for the intraperitoneal transverse and sigmoid colons, which are
tethered to the posterior abdominal wall by mesocolons.
The sigmoid colon joins the rectum in the pelvis, near the third sacral
vertebra. The rectum is the final 8 inches (20 cm) of the alimentary
canal. It extends anterior to the sacrum and coccyx. Even
though rectum is Latin for "straight," this structure has three lateral
bends that create a trio of internal transverse folds called the rectal
valves. These valves allow passing of gas (flatus) while preventing the
simultaneous passage of feces.
The last part of the large intestine is the anal canal, which is located in
the perineum, completely outside the abdominopelvic cavity. This 1 inch
(3 cm) long structure opens to the exterior of the body at the anus. The
anal canal includes two sphincters. The involuntary internal anal
sphincter is made of smooth muscle; the voluntary external anal
sphincter is skeletal muscle. Except when defecating, these two
sphincters are usually closed.