Physiology And Pharmacology Of The Large Bowel

Question 1

Where does majority of water absorption occur in the colon?

Answer 1

Jejunum

Question 2

Where does the remaining water reabsorption occur in the large colon?

Answer 2

Ascending colon and large colon.

Question 3

How does the colon compensate for the small intestine dysfunction?

Answer 3

Dysfunction of the small intestine reduces reabsorption of material, so more material passes into the large colon, therefore the ascending colon of the large intestine will try to compensate by reabsorbing more water, therefore more material will be lost as faeces.

Question 4

How does the colon compensate for the large intestine dysfunction?

Answer 4

Small intestine is still functional, therefore, majority of material will be reabsorbed however there will be reduction in water and electrolyte absorption which means more material will be lost as faeces.

Question 5

What is peristalsis?

Answer 5

Muscles squeeze material along the digestive tract.

Question 6

What is segmentation?

Answer 6

Muscle activity in the colon breaking up the material in the intestines.

Question 7

Which materials does the colon secrete?

Answer 7

Parasympathetic nervous system promotes the secretion of a mucosal layer for immune defence against material in food.

Question 8

How is water absorbed by the large intestine?

Answer 8

Promoting salt absorption via the Na+K+ exchanger from faecal material in enterocytes to increase intracellular salt concentration, which lowers the intracellular osmotic potential that draws water in along with Cl-. Aldosterone increases the action of intracellular sodium channels

Question 9

What causes constipation?

Answer 9

Too much water absorption.

Question 10

What causes diarrhoea?

Answer 10

Too little water absorption.

Question 11

What is the innervation of the colon?

Answer 11

Branches of:
Vagus nerve and Pelvic nerve.

Question 12

Which portion of the colon is innervated by the vagus nerve?

Answer 12

Oesophagus, stomach, small intestine, and the large intestine up to the splenic flexure of the colon.

Question 13

Which portion of the colon is innervated by the pelvic nerve?

Answer 13

Pelvic nerve innervates the distal 1/3 of transverse colon to the sigmoid colon, to provide parasympathetic innervation from branches of the inferior mesenteric plexus.

Question 14

What is the sympathetic innervation of the colon?

Answer 14

Lumbar splanchnic nerve is a branch of the inferior mesenteric plexus which innervates the distal 1/3 of the transverse colon to the rectum.

This is in contrast to the parasympathetic supply by the pelvic splanchnic nerve.

Question 15

What are the phases of defaecation?

Answer 15

Holding position
First stage of defaecation
Second stage of defaecation

Question 16

What triggers the defaecation reflex?

Answer 16

Distention of the rectal wall accompanied by the conscious urge to defaecation.

Question 17

What is the holding position of defaecation?

Answer 17

Contraction of the external anal sphincter and the puborectalis muscle to hold foecal material in the rectum.

Question 18

What is the first stage of defaecation?

Answer 18

Relaxation of the puborectalis and external anal sphincter.
Contraction of the levator ani, diaphragm and rectus muscles to increase intraabdominal pressure.

Question 19

What second stage of defaecation?

Answer 19

Relaxation of the internal anal sphincter alongside rectal extraction to expel faeces.

Question 20

What is the sympathetic control of the GI tract?

Answer 20

Inhibits peristalsis for digestion
Inhibits contraction of the bladder and rectum.

Question 21

What is the parasympathetic control of the GI tract?

Answer 21

Increases peristalsis of the GI tract
Relaxation of the bladder and rectum.

Question 22

What drives the movement of the faeces in the colon?

Answer 22

Activation by the myenteric plexus in the colon and rectum, which will receive parasympathetic innervation from the pelvic splanchnic nerves to the taenia Coli and smooth muscle of the colon which create large peristaltic movements, that lead to distention of the rectum that triggers the defaecation reflex.

Question 23

What is essential for the expulsion of faeces?

Answer 23

Relaxation of the external anal sphincter, via the somatic branch of the pudendal nerve.

Question 24

What is the major composition of faeces?

Answer 24

Water makes up the majority, followed by dead bacteria and fat.

Question 25

What cases the colour of faeces?

Answer 25

Stercobilin and Urobilib, derivatives from bilirubin from the breakdown of haem.

Question 26

What is used to categorise stool consistency?

Answer 26

Bristol Stool Chart from a scale of 1-7, with 1 being hard lumps and 7 being watery and liquid stools.

Question 27

What is the ideal rating for stool on the Bristol Stool Chart?

Answer 27

Type 3/4 stool, which is sausage shaped and cracked/soft.

Question 28

What rating on the Bristol Stool chart indicate constipation?

Answer 28

Type 1 or Type 2 stool.

Question 29

What rating on the Bristol Stool Chart indicate diarrhoea?

Answer 29

Type 5-7 stool.

Question 30

What is the cause of constipation?

Answer 30

Defaecation is delayed which leads to faeces drying, that comes with a risk for appendicitis. It is caused by obstruction, ignoring urge to defaecation or reduced colon motility due to aging.

Question 31

How is IBD treated?

Answer 31

Ulcerative colitis and Crohn’s disease are treated with aminocsalicylates such as sulphasalazine and mesasalazine.

Question 32

How does sulphalazine work?

Answer 32

It is an immunomodulator prodrug which is converted into its active form in the large intestine by colonic bacteria. The 5-ASA portion inhibits the production of inflammatory mediators such as NFK-beta and the COX pathway and Lipo-oxygenase pathway.

A portion of it is taken up into the bloodstream and cause systemic effects with blood disorders, nausea and rashes due to hypersensitivity.

Question 33

How does Mesasalzine work?

Answer 33

Immunoomoduator which acts directly on colonic mucosa to decrease inflammatory mediator production and leukotriene synthesis. It impacts the kidneys and is affected by treatments that affect gastric pH, which affect the distribution of the drug.

Question 34

What is the treatment for IBS?

Answer 34

Anti-spasmodic drugs which inhibit the action of acetylcholine on intestinal smooth muscle to prevent the contraction that causes abdominal pain and cramping.

Question 35

What is the action of loperamide?

Answer 35

Potent opioid agonist of mu receptors on the smooth muscle of the GI tract to decrease peristalsis and increase time for water absorption to increase stool thickness and decrease diarrhoea. It is an anti-motility drug, and as an opioid, may affect driving and operating machinery.

Should be avoided in:
bacterial diarrhoea, which may increase colonisation
Ulcerative colitis due to increased risk of perforating megacolon with slower transit time

Question 36

What is the action of codeine phosphate?

Answer 36

Anti-motility drug which acts on mu opioid receptors in the GI tract to reduce transit time. There is a potential for dependence and toxicity.

Question 37

What is the action of co-phenotrope?

Answer 37

Consists of diphenoxylate combined with atropine, an anti-cholinergic medication to prevent abuse by overdosage.

DIphenoxylate is an opioid agonist which acts on mu opioid receptors to increase transit time. It is able to cross the blood brain barrier, therefore there is a risk of overdosage which is greater in young children. It has very little anti-analgesic activity.

Question 38

What is the action of kaolin?

Answer 38

Metal compound which adsorbs onto water in stools to act as a binding agent to reduce diarrhoea.

Question 39

What is kaolin prescribed with?

Answer 39

Morphine which binds to opioid receptors to cause decreased peristalsis and increased water absorption
OR
Calcium carbonate, an antacid which reduces the levels of gastric acid in the stomach and reduces peristalsis.

Question 40

What is Isphagala husk?

Answer 40

A bulk-forming laxative which is hydrophilic and binds to stools to increase the weight by stretching receptors in the mucosa that increases transit time when fibre in diet cannot be increased. It can reduce cholesterol levels.

Question 41

What is an osmotic laxative?

Answer 41

Lactulose, a synthetic dissacharide which releases osmotically active sugars to increase the movement of water into the stools. Lactulose decreases ammonia levels and causes acidification of stools, making it useful for hepatic encephalopathy to prevent ammonia buildup.

Should be avoided in patients with heart failure or electrolyte disturbances.

Question 42

What is a stimulant laxative?

Answer 42

Senokot which increases the activation of the myenteric plexus to increase gastric motility and reduce water absorption time. Senokot takes 8-12 hours to have an effect and can be combined with Isphagala husk. However, it can cause abdominal cramps.

Question 43

What is another stimulant laxative?

Answer 43

Bisacodyl which stimulates the myenteric plexus for parasympathetic activation of the colon to increase peristalsis and motility. It reduces water absorption and takes 6-12 hrs to become effective, used before surgery or radiological examinations.

Bisacodyl reduces stomach acidity

Question 44

What are the risks with laxative use?

Answer 44

Melanosis Coli
Tolerance

Question 45

What is melanosis coli?

Answer 45

Pigmentation of the colon with brown/black discolouration, that is typically asymptomatic and reversible with cessation of laxative use.