Physiology and Pharmacology of the Large Intestine

Question 1

When does stuff enter the caecum?

Answer 1

After the gastroileal reflex (in response to gastrin and CCK).

Question 2

What signals is the ileocaecal valve controlled by?

Answer 2

The vagus nerve, sympathetic nerves, enteric neurones and hormonal signals.

Question 3

What is the opening to the appendix called?

Answer 3

The appendiceal orifice.

Question 4

What is absorbed in the large intestine?

Answer 4

Sodium, chloride, water, short chain fatty acids (from carbohydrate fermented by colonic flora).

Question 5

What is secreted in the large intestine?

Answer 5

Potassium, bicarbonate and mucus.

Question 6

What are the main functions of the ascending and transverse colon?

Answer 6

Fluid reabsorption and bacterial fermentation.

Question 7

What are the main functions of the descending and sigmoid colon?

Answer 7

Final drying and storage.

Question 8

What are present in the colon to increase surface area?

Answer 8

Colonic folds, crypts and microvilli (no villi).

Question 9

What cells mediate electrolyte absorption?

Answer 9

Colonocytes.

Question 10

What cells mediate ion secretion?

Answer 10

Crypt cells.

Question 11

What do goblet cells do in the large intestine?

Answer 11

Secrete lots of mucus containing glycosaminoglycans (hydrated to form a slippery gel), trefoil proteins involved in host defence.

Question 12

What is enhanced by aldosterone?

Answer 12

Sodium absorption and potassium secretion.

Question 13

What are the 3 patterns of motility in the large intestine?

Answer 13

Haustration (non-propulsive segmentation), peristaltic propulsive movements (mass movement), defecation (periodic egestion).

Question 14

What are haustra?

Answer 14

Saccules cause by alternating contraction of the circular muscle.

Question 15

What is the main difference between segmentation and haustration?

Answer 15

Haustration has a much lower frequency.

Question 16

What are the functions of haustration and what is it generated by?

Answer 16

It causes oral movement, allows time for fluid and electrolyte reabsorption, mixes content. It is generated by slow wave activity.

Question 17

What is mass movement?

Answer 17

Simultaneous contraction of large sections of the circular muscle of the ascending and transverse colon (haustra disappear) which drives faeces into distal regions.

Question 18

How often does mass movement occur and what is it triggered by?

Answer 18

About 1 to 3 times daily, triggered by a meal via the gastrocolic response.

Question 19

What mediators does the gastrocolic reponse involve?

Answer 19

Gastrin and extrinsic nerve plexuses.

Question 20

What happens when mass movement occurs in the distal colon?

Answer 20

It propels faeces into the rectum triggering the defecation reflex in response to stretch.

Question 21

What happens when rectal stretch receptors are activated?

Answer 21

Causes contraction of smooth muscle of sigmoid colon and rectum - internal anal sphincter relaxes.

Question 22

Once the sigmoid colon and rectum have contracted, what are the 2 options?

Answer 22

1. Relaxation of skeletal muscle of external anal sphincter.
2. Contraction of skeletal muscle of external anal sphincter.

Question 23

What is defection assisted by?

Answer 23

Straightening of the anorectal angle (squatting), abdominal skeletal muscle contraction and expiration against a closed glottis.

Question 24

What do gut bacteria synthesise?

Answer 24

Vitamin K2 and free fatty acids (from carbohydrate).

Question 25

What are all the causes of conspitation?

Answer 25

Ignoring/suppressing the urge to defecate, decreased colonic motility, obstruction of faecal movement, paralytic ileus following abdominal surgery, impairment of motility/defecation reflex (Hirshprung disease).

Question 26

What symptoms can constipation cause?

Answer 26

Abdominal discomfort, headache, loss of appetite and general malaise.

Question 27

What are the symptoms of constipation caused by and what are they not caused by?

Answer 27

Cause by prolonged distention of the large intestine, not toxins absorbed from retained faecal matter.

Question 28

Name a metabolic disease causing decreased motility.

Answer 28

Hypokalaemia.

Question 29

When should laxatives or purgatives not be used?

Answer 29

When there is physical obstruction to the bowel.

Question 30

What do laxatives do?

Answer 30

Increase peristalsis and/or soften faeces causing, or assisting evacuation.

Question 31

What can cause laxative dependency?

Answer 31

Using laxatives too regularly where you develop an atonic colon.

Question 32

What are clinical indications of laxatives/purgatives?

Answer 32

When straining is potentially damaging to health e.g. angina or defecation is painful e.g. haemorrhoids, to clear the bowel before surgery or endoscopy, to tread drug-induced constipation or constipation in bedridden or elderly patients.

Question 33

What are the 4 classes of laxative?

Answer 33

Bulk, osmotic, stimulant, faecal softeners.

Question 34

What are bulk laxatives and are they fast or slow acting?

Answer 34

Indigestible polysaccharide polymers. Slow acting.

Question 35

Give an example of a bulk laxative.

Answer 35

Methylcellulose.

Question 36

What are osmotic laxatives and are they fast or slow acting?

Answer 36

Poorly absorbed solutes, fast acting.

Question 37

Give 3 examples of osmotic laxatives and how they are taken.

Answer 37

Magnesium sulphate or hydroxide orally, sodium citrate rectally, lactulose orally.

Question 38

What is the function of stimulant purgatives?

Answer 38

They cause increased water and electrolyte secretion and increased peristalsis.

Question 39

Give 3 examples of stimulant purgatives.

Answer 39

Bisacodyl, sodium picosulphate, senna.

Question 40

What is a common side effect of stimulant purgatives?

Answer 40

Abdominal cramps.

Question 41

How do faecal softeners work?

Answer 41

They have a detergent-like action.

Question 42

Give 3 examples of faecal softeners and how they are taken.

Answer 42

Docusate sodium orally, arachis oil as an enema.

Question 43

What is the treatment for IBS?

Answer 43

Adjustment of diet, anti-diarrhoeals, anti-spasmodics or laxatives as required.