Download Digestion

The Digestive
System
Chapter 23
Anatomy of the Digestive
System – Part 4
Large Intestine
• Frames the small intestine on 3
sides
• Extends from the ileocecal valve
to the anus.
• Lumen diameter ~7cm (2 ¾ in)
• Length 1.5 meter (5 ft)
Large Intestine
• Major function is to absorb most of the
remaining water from the indigestible food,
stores the residue temporarily, and then
eliminate them from the body in the form of
feces.
• Because the colon doesn’t secrete digestive
enzymes, chemical digestion officially ends in
the small intestine.
Large Intestine
• Cecum (“blind pouch”) – lies below the
ileocecal valve. First part of the large intestine.
Large Intestine
• Appendix – attached to the cecum. Contains
lymphoid tissue and plays an important role in
body immunity
Large Intestine
• Ascending colon – travels up the right side of
the abdominal cavity
Large Intestine
• Transverse colon – travels across the
abdominal cavity
Large Intestine
• Descending colon – travels down the left side
of the abdominal cavity
Large Intestine
• Sigmoid colon – when colon enters the pelvis,
s-shaped, travels transverse
Large Intestine
• Rectum – travels down, just in front of the
sacrum
Large Intestine
• Anal canal – last portion of large intestine,
about 3 cm long. Has the sphincters that open
and close the anus during defecation
Large Intestine
• Since most food is absorbed before reaching
the large intestine, there are no circular folds,
no villi, and virtually no cells that secrete
digestive enzymes.
Large Intestine
• The mucosa is thicker, and abundant crypts
are deeper and there are lots of goblet cells
that produce mucus. The mucus eases the
passage of feces and protects the intestinal
wall from irritating acids and gases released by
resident bacteria
Large Intestine - Bacterial Flora
• Most bacteria entering from the small
intestine are dead, but some are still alive.
These and the bacteria that enter the GI tract
from the anus, these constitute the bacterial
flora of the large intestine over 700 species!
Large Intestine - Bacterial Flora
• Metabolize some proteins and ferment some
indigestible carbohydrates (cellulose, xylan,
and others), releasing irritating acids and a
mixture of gases (including dimethyl sulfide,
H2, N2, CH4, and CO2). Some are quite odorous
(dimethyl sulfide)!
Large Intestine - Bacterial Flora
• About 500 ml of gas (flatus) is produced each
day. More when certain carbohydrate rich
foods (beans) are eaten.
Large Intestine - Bacterial Flora
• Bacteria also synthesize B complex vitamins
and most of the vitamin K that the liver needs
to make some of the clotting proteins.
• Also found in the large intestine are viruses
and protozoans. Of these at least 20 are
known pathogens.
Large Intestine - Bacterial Flora
• Most enteric bacteria are
commensals (no harm and no
benefit to the host) that coexist
peacefully with their host as long
as they remain in the lumen.
• Bacteria break down and
metabolize the residual complex
carbohydrates further, adding
much to their own nutrition but
essentially nothing to ours.
Large Intestine - Motility
• Haustral contractions
– Slow segmenting
movements mainly
occuring in the
transverse and
descending colon.
– Last about 1 minute and
occur every 30 minutes
or so
Large Intestine - Motility
• Mass movements
– Long, slow-moving, powerful contractile waves
that move over large areas of the colon 3-4 times
daily and force the contents towards the rectum.
– Typically occurs during or just after eating.
Homeostatic Imbalance - Diverticulosis
• Bulk, or fiber, in the diet increases the
strength of colon contraction and softens
the stool, allowing the colon to act like a
well-oiled machine.
• When the diet lacks bulk, and the volume
of the residues in the colon is small, the
colon narrows and its contractions
become more powerful, increasing the
pressure on its walls. This promotes the
formation of diverticula, small
herniations of the mucosa through the
colon walls.
• Most commonly occurs in the sigmoid
colon. Affects over half of people over
the age of 70.
Defecation
• The semisolid product delivered to the rectum
is called feces or stool, contains undigested
food residues, mucus, sloughed-off epithelial
cells, millions of bacteria, and just enough
water to allow for smooth passage.
• The rectum is usually empty, but when mass
movements force the feces into it, the
defecation reflex is triggered.
Homeostatic Imbalance - Diarrhea
• Watery stools
• Results from any condition that
rushes food residue through the
large intestine before that organ
has had sufficient time to absorb
the remaining water.
• Prolonged diarrhea may result in
dehydration and electrolyte
imbalance.
Homeostatic Imbalance - Constipation
• When food remains in the colon
for extend periods, too much
water is absorbed and the stool
becomes hard and difficult to
pass.
• May result from a lack of fiber in
the diet, improper bowel habits
(holding it), lack of exercise,
emotional upset, or laxative
abuse