Download Large Intestine Notes

Biology 2122
Notes: Chapter 23
Large Intestine/Digestion and Absorption
Topic
Gross Anatomy
of the Large
Intestine
Notes
a). There are three structures not seen in any other part of the GI tract
anatomy. The longitudinal muscle layer transitions into smooth muscle
called teniae coli and the contractions in this muscularis area causes the
wall to bulge out and this is called haustra. The epiploic appendages
are fat-filled pouches of the visceral peritoneum that hang from the
outer surface of the intestine.
b). Make sure you know each of the following: cecum, vermiform
appendix, parts of the colon (ascending, transverse, etc.), sigmoid colon,
mesocolon, etc.
Microscopic
anatomy
c).The sigmoid colon joins the rectum. The rectal valves are transverse
folds which separate feces from being passed along with gas. The final
part, the anal canal, is the last segment. It is approximately 3 cm long
and contains two important sphincters. The involuntary internal anal
sphincter is made of smooth muscle and the external sphincter is made
of skeletal muscle.
a). Differences in the wall of the large intestine are:
 Mucosa is simple columnar epithelium except in the anal canal
(stratified squamous)
 No villi, no digestive secreting cells, no absorption takes place.
 Thick mucus, a great deal of globlet cells. Allows for smooth
passage of feces and protects against bacteria.
b). Teniae coli and haustra are not present in the rectum and anal canal.
c). Flora: Those bacteria that survive the upper GI tract are now called
bacterial flora. About 700 species. Their roles are to:
 Metabolize proteins
 Ferment some indigestible carbohydrates
 Release irritating acids and gases such as dimethyl sulfide
 Synthesis of B and K vitamins
 Some viruses and protozoans
d). How does the large intestine keep from being infected all the time?
The epithelial cells release chemicals that recruit immune cells into the
mucosa and pry open the epithelial cells and send extensions to the
lumen to sample the bacteria or pathogens. Migrate to the MALT area
Digestive
Processes
and present antigens to the T cells.
a). Food materials stay for approximately 12 – 24 hours. Most water is
reclaimed but some are absorbed by fecal material. Remember absorption
is not the main function of the large intestine.
b).Contractions are very slow and long. Haustral contractions are most
seen. They last for about 30 minutes. Mainly in transverse and
descending colon. Mass movements are long and powerful. They occur 34 times a day. Occur after eating.
c). Diverticulosis is a condition which occurs in the sigmoid colon. It occurs
when the diet lacks bulk and the volume is low. This causes more powerful
contractions applying very great pressure to the walls causing small
hernias. A more serious condition called divertculitis occur when the
diverticula (small herniations) become inflamed and may rupture.
d). Defecation: When feces reaches the rectum via mass movements, a
defecation reflex occurs:
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 Parasympathetic reflex causes the rectum and sigmoid to contract
and the internal sphincter to relax. Messages are relayed to the
brain and then the external sphincter is voluntarily relaxed. The
following mass movement causes defecation.