Large Intestine consist of cecum, appendix, colon [ascending
colon, transverse colon and descending colon, end part of
descending colon forms sigmoid colon] and rectum. Large Intestine
does the function of water and electrolyte absorption and works as
storage organ [primary function of large intestine is to store
feces]. 2
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The colon normally receives 500ml of Chyme from the Small
Intestine each day. The contents coming to the colon consist of -
Indigestible food residues e.g. cellulose, Unabsorbed Biliary
components,Fluid. - Colon absorbs water and salt, what remains
behind is to be eliminated is know as feces. 4
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We will discuss 1. Motility 2. Secretion 3. Digestion 4.
Absorption MOTILITY IN LARGE INTESTINE Haustral Contraction or
Segmentation Peristaltic wave Mass movement [strong peristaltic
waves] 5
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Haustral Contraction [Segmentation Contraction] They help to
mix the contents of colon and expose contents to mucosa to
facilitate absorption. They occur less frequently may be after
every 30mins [they are like segmentation contraction in small
intestine but in small intestine they occur 10- 12/min]. Haustral
contraction are largely controlled by locally mediated reflexes
involving the intrinsic plexus. 6
Slide 7
Peristaltic wave They propel the contents towards the rectum.
Mass movement [strong peristaltic waves] They move the material
from one portion of intestine to another. They occur 3-4 times per
day, generally after meals and increase in motility moves the feces
forward in few seconds. When material reaches the rectum, rectal
distention initiates the defecation reflex. 7
Slide 8
Mass movement [strong peristaltic waves] Gastro-colic Reflex
when food enters the stomach, mass movements are triggered in the
colon by gastro-colic reflex also. It is mediated from stomach to
the colon by gastrin and extrinsic autonomic nerves. It pushes the
colonic contents into rectum triggering the defecation reflex.
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SECRETION Large intestine secretes alkaline NaHCO 3, mucus
solution. Its function is to protect large intestine from
mechanical and chemical injury. Mucus provides lubrication to
facilitate the passage of feces. 9
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DIGESTION There are no digestive enzymes secreted, therefore,
no digestion takes place in large intestine. 10
Slide 11
ABSORPTION Na+ is actively transported and water follows along
the osmotic gradient. Secretion of K+ and HCO 3. Due to absorptive
capacity, some drugs are given per rectum especially in children
e.g. anesthetics, steroids. There are no villi in Large Intestine.
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About 500ml of material entering the colon per day from the
small intestine, colon absorbs about 350ml, leaving 150g of feces
to be eliminated per day. Feces contains 100g of water and 50g of
solid [undigested cellulose, bilirubin [stercobilinogen], bacteria,
unabsorbed food residue. 12
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They are commensals, which have no effect on host [they are not
pathogen which cause disease]. Bacteria present are E.coli,
bacteroides fragilis. Some bacteria synthesize vitamin K, vitamin
B- complex, folic acid. 14
Slide 15
Feces are eliminated by defecation reflex. How this reflex
works? When mass movements of colon move the feces into the rectum.
Distention of rectum initiates the reflex. Stretch receptors in the
wall of rectum send impulses to the spinal cord ( S2,S3,S4),
parasympathetic nerves causes contraction of smooth muscle of
rectum and sigmoid colon and relaxation of internal sphincter.
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If external anal sphincter [which is skeletal muscle is also
relaxed defecation occurs]. External anal sphincter is under
voluntary control, therefore, can prevent defecation despite
defecation reflex. 16
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When defecation occurs, it is assisted by voluntary straining
movements that involve contraction of abdominal muscles and
forceful expiration against closed glottis which increases
intra-abdominal pressure. 17
Slide 18
When more water is absorbed from the feces, they become hard
and dry. Normally frequency of passing stool vary. It maybe once a
day, or after every meal or once every 2 or 3 days. CAUSES OF
CONSTIPATION Decreased colonic motility due to low bulk diet,
aging, emotion, anxiety. Colonic spasm, tumor in colon. Injury to
nerve pathway. 18
Slide 19
It is derived from two sources 1. Swallowed air [up to 500ml of
air may be swallowed during a meal]. 2. Gas produced by bacterial
fermentation in the colon. Most gas in the colon is due to result
of bacterial activity, but the quantity and the nature of gas
produced depend on the type of food eaten and colonic bacteria.
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Food such as beans, contain carbohydrate that human can not
digest but can be attacked by gas producing bacteria. Gases
produced are Hydrogen, Hydrogen Sulphide, Nitrogen, Carbon dioxide
and Methane. 20
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Amount of gas per day passed is about 200ml. The smell is
largely due to sulphides. Gas passing through the luminal contents
give rise to gurgling sounds known as BORBORGYMI. 21
Slide 22
We will discuss the following hormones: - Gastrin - Secretin -
CCK - Motilin - Somatostatin - GIP - VIP 22
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Produced by G-cells in the stomach. Stimulates the release of
HCL and Pepsinogen in the stomach. Increases gastric motility.
Increases ileal motility. Relaxes Ileocecal Sphincter. Induces mass
movements in colon [because to help the contents moving through GIT
on arrival of new meal in the stomach]. 24
Slide 25
Secretin is released from small intestine when stomach acid
contents come to duodenum. Functions It inhibits gastric emptying
to delay the acid contents of stomach to enter in the duodenum. It
inhibits gastric secretion. It acts on pancreatic duct to produce
large volume of watery, NaHCO 3 secretion. It stimulates the
secretion of NaHCO 3 rich bile in the liver by acting on the bile
ducts. 25
Slide 26
CCK is released from the duodenum in response mainly to fat, to
a lesser extent to protein products. CCK causes (i). Inhibition of
Gastric motility and secretion (ii). Stimulates pancreatic Acinar
cells to secrete pancreatic enzymes [amylase, lipase, Trypsinogen,
Chymotrypsinogen]. (iii). It causes contraction of gall-bladder and
relaxation of sphincter of Oddi. 26
Slide 27
It is polypeptide and secreted by entrochromaffin cells and Mo
cells in the stomach, small intestine and colon. It causes
contraction of smooth muscles in the stomach and intestine. Its
level increases during inter-digestive state and controls GIT
motility, Migrating motility complex (MMC) between the meals.
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Slide 28
It is secreted by D-Cells in pancreatic islets and by similar
D-cells in GIT mucosa. Somatostatin inhibits secretion of Gastrin,
VIP, GIP, Secretin and Motilin. Somatostatin secretion is
stimulated by acid in the lumen of intestine. It acts probably in a
paracrine fashion. 28
Slide 29
GIP is released from the duodenum. GIP causes release of
insulin. Insulin causes uptake and storage of glucose. GIP before
it was called Gastric Inhibitory Peptide [this role is minimum].
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Slide 30
VIP is found in nerves in the GIT. It stimulates intestinal
secretion of electrolytes and water. Other action relaxation of
intestinal smooth muscle including sphincters. Inhibition of
gastric acid secretion. It potentiates the action of acetylcholine
in salivary glands. VIP is also found in brain, blood, autonomic
nerves. 30
