Small Intestine
The small intestine extends from the pyloric sphincter to the ileocecal
valve, where it empties into the large intestine. The small intestine
finishes the process of digestion, absorbs the nutrients, and passes
the residue on to the large intestine. The liver, gallbladder, and
pancreas are accessory organs of the digestive system that are closely
associated with the small intestine.
The small intestine is divided into the duodenum, jejunum, and
ileum. The small intestine follows the general structure of the
digestive tract in that the wall has a mucosa with simple columnar
epithelium, submucosa, smooth muscle with inner circular and outer
longitudinal layers, and serosa. The absorptive surface area of
the small intestine is increased by plicae
circulares, villi, and microvilli.
Exocrine cells in the mucosa of the small intestine secrete mucus,
peptidase, sucrase, maltase, lactase, lipase, and enterokinase.
Endocrine cells secrete cholecystokinin and secretin.
The most important factor for regulating secretions in the small
intestine is the presence of chyme. This is largely a local reflex
action in response to chemical and mechanical irritation from the
chyme and in response to distention of the intestinal wall. This
is a direct reflex action, thus the greater the amount of chyme,
the greater the secretion.
Large Intestine
The large intestine is larger in diameter than the small intestine.
It begins at the ileocecal junction, where the ileum enters the
large intestine, and ends at the anus. The large intestine consists
of the colon, rectum, and anal canal.
The wall of the large intestine has the same types of tissue that
are found in other parts of the digestive tract but there are some
distinguishing characteristics. The mucosa has a large number of
goblet cells but does not have any villi. The longitudinal muscle
layer, although present, is incomplete. The longitudinal muscle
is limited to three distinct bands, called teniae coli, that run
the entire length of the colon. Contraction of the teniae coli exerts
pressure on the wall and creates a series of pouches, called haustra,
along the colon. Epiploic appendages, pieces of fat-filled connective
tissue, are attached to the outer surface of the colon.
Unlike the small intestine, the large intestine produces no digestive
enzymes. Chemical digestion is completed in the small intestine
before the chyme reaches the large intestine. Functions of the large
intestine include the absorption of water and electrolytes and the
elimination of feces.
Rectum and Anus
The rectum continues from the signoid colon to the anal canal and
has a thick muscular layer. It follows the curvature of the sacrum
and is firmly attached to it by connective tissue. The rectum and
ends about 5 cm below the tip of the coccyx, at the beginning of
the anal canal.
The last 2 to 3 cm of the digestive tract is the anal canal, which
continues from the rectum and opens to the outside at the anus.
The mucosa of the rectum is folded to form longitudinal anal columns.
The smooth muscle layer is thick and forms the internal anal sphincter
at the superior end of the anal canal. This sphincter is under involuntary
control. There is an external anal sphincter at the inferior end
of the anal canal. This sphincter is composed of skeletal muscle
and is under voluntary control.
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