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LARGE INTESTINE

DR. AMEL EASSAWIDR. SHAIKH MUJEEB AHMED

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OBJECTIVESThe student should be able to: Outline the different parts of the large Intestine. Describe motility, secretion, digestion, absorption in

the large intestine. Know the functions of Intestinal bacteria. Describe feces, defecation reflex, flatus and

constipation.

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LARGE INTESTINE• 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].

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LARGE INTESTINE• The colon normally receives 500 ml 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.

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LARGE INTESTINEMotility in Large Intestine• Haustral contraction or segmentation • Mass movement [strong peristaltic waves]

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MOTILITY IN LARGE INTESTINEHaustral 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 30

mins [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.

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MOTILITY IN LARGE INTESTINE

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.

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MOTILITY IN LARGE INTESTINE

Mass Movement [Strong Peristaltic Waves]• Gastro-colic Reflex – when food enters the

stomach, mass movements are triggered in the colon by gastro-colic reflex.

• 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 AND DIGESTION IN LARGE INTESTINE

Secretion• Large intestine secretes alkaline NaHCO3, mucus

solution.• Its function is to protect large intestine from

mechanical and chemical injury.• Mucus provides lubrication to facilitate the passage

of feces.

Digestion• There are no digestive enzymes secreted, therefore,

no digestion takes place in large intestine.

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ABSORPTION INLARGE INTESTINE

Absorption• Na+ is actively transported and water follows

along the osmotic gradient.• Secretion of K+ and HCO3.• 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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FECES• About 500 ml of material entering the colon per

day from the small intestine, colon absorbs about 350 ml, leaving 150 gram of feces to be eliminated per day.

• Feces contains 100 gram of water and 50 gram of solid [undigested cellulose, bilirubin [stercobilinogen], bacteria, unabsorbed food residue.

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LARGE INTESTINE BACTERIA• They have no effect on host [they are not

pathogen].• Bacteria present are E.coli, bacteroides –

fragilis. • Some bacteria synthesize vitamin K, vitamin B-

complex, folic acid.

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DEFECATION REFLEX• 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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DEFECATION REFLEX• 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.

• 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.

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CONSTIPATION• 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.

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INTESTINAL GAS OR FLATUS• It is derived from two sources: 1. Swallowed air [up to 500 ml 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.

• 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.

• Amount of gas per day passed is about 200 ml. The smell is largely due to sulphides.

• Gas passing through the luminal contents give rise to gurgling sounds known as borborgymi.

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ANATOMY AND FUNCTIONS OF THE DIGESTIVE SYSTEM

REFERENCES

• Human physiology by Lauralee Sherwood, seventh edition

• Text book physiology by Guyton &Hall,11th edition• Text book of physiology by Linda .s contanzo,third

edition

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