Physiology of the large intestine

Question 1

What are the 3 functions of the large intestine?

Answer 1

To extract Na+ and water from the luminal contents

To make and store faeces

To move faeces towards the rectum

Question 2

What is the only secretion of the large intestine and which cells secrete it?

Answer 2

Alkaline mucus - secreted by goblet cells

Question 3

What 2 functions does alkaline mucus have in the large intestine?

Answer 3

Protects epithelium from acid, abrasion, and bacterial activity

Provides an adherent medium for holding faecal matter together

Question 4

How much water is lost in faeces each day?

Answer 4

100ml

Question 5

Where are tight junctions most leaky - duodenum or colon?

Answer 5

Duodenum

Question 6

Where are tight junctions most tight - duodenum or colon?

Answer 6

Colon

Question 7

In the duodenum and jejunum, is water absorbed via a paracellular or transcellular route?

Answer 7

Paracellular

Question 8

In the colon, is water absorbed via a paracellular or transcellular route?

Answer 8

Transcellular

Question 9

What are the 2 specialised movements of the colon?

Answer 9

Haustral contractions

Mass movements

Question 10

What are haustral contractions?

Answer 10

Localised segmented contractions of the circular muscles in the proximal colon, leading to mixing of the luminal content without moving it in any particular direction

Question 11

What is the frequency that mass movements occur?

Answer 11

1-3 times a day

Question 12

Mass movements are initiated by what?

Answer 12

Gastrocolic reflex

Question 13

The gastrocolic reflex is mediated by which 2 things?

Answer 13

Gastrin and extrinsic autonomic nerves

Question 14

What is the defecation reflex initiated by?

Answer 14

Distension of the rectum from faeces

Question 15

Which mechanism is a voluntary method of initiating defecation?

Answer 15

Valsalva manoeuvre

Question 16

Describe the valsalva manoeuvre.

Answer 16

Full inspiration followed by forced expiration against a closed glottis.

Diaphragm moves downwards, abdominal muscles contract, increasing abdominal pressure.

Increased pressure in abdomen forces faecal contents into the rectum, initiating defecation reflex.

Question 17

Faecal odour is caused by what?

Name 2 specific compounds which contribute to odour.

Answer 17

Products of bacterial action

Skatole
Hydrogen sulphide

Question 18

List some functions of intestinal bacteria.

Answer 18

Immunity against other bacteria

Convert bilirubin to urobilinogen

Form secondary bile acids

Digest enzymes and mucus

Synthesize certain vitamins (e.g. vit K)

Metabolise undigested polysaccharides