Physiology of the Large Intestine

Question 1

What are the 3 functions of the large intestine?

Answer 1

1. To extract Na+ and water from the luminal contents.
2. To make and store faeces.
3. To move faeces towards the rectum.

Question 2

What is the only secretion of the large intestine?

Answer 2

An alkaline mucus from Goblet cells.

Question 3

What functions does the secretion of alkaline mucus from the large intestine have?

Answer 3

1. Protects the epithelium from acid, abrasion, and bacterial activity.
2. Provides an adherent medium for holding faecal matter together.

Question 4

In the intestines, what is the driving force for water absorption?

Answer 4

The movement of Na+.

Question 5

How does Na+ enter the luminal membrane of the intestinal epithelium of the jejunum?

Answer 5

On coupled transporters e.g. with sugars or amino acids.

Question 6

How does Na+ leave the jejunal epithelial cells on the basolateral membrane?

Answer 6

Via Na+-K+ATPase

Question 7

Which junctions are between the epithelial cells of the intestine?

Answer 7

Tight junctions.

Question 8

Going from the duodenum to the colon, how do the tight junctions changes?

Answer 8

They are least tight/more leaky in the duodenum and tightest/least leaky in the colon.

Question 9

How is the route that water takes different in the duodenum to the colon?

Answer 9

In the duodenum the movement of water is paracellular, whereas in the colon it is transcellular.

Question 10

What separates the terminal end of the ileum from the caecum?

Answer 10

The Ileocaecal Valve.

Question 11

What are the functions of the Ileocaecal valve?

Answer 11

1. Prevents back-flow of faecal contents from the colon.

2. Controls the rate at which the ileal chyme enters the colon.

Question 12

What are the types of movements of the colon?

Answer 12

1. Haustral contractions

2. Mass movements

Question 13

What are the functions of the motility of the colon?

Answer 13

1. Facilitates the efficient absorption of water and salts.

2. Permits the orderly evacuation of faeces.

Question 14

Which type of motility occurs in the proximal part of the colon?

Answer 14

Haustral contractions

Question 15

Which type of motility occurs in the distal part of the colon?

Answer 15

Mass movements

Question 16

How often do mass movements occur per day?

Answer 16

1-3 times per day.

Question 17

What are the reflexes associated with defecation?

Answer 17

1. The Gastrocolic reflex

2. The Defecation reflex

Question 18

What is the Gastrocolic reflex mediated by?

Answer 18

Gastrin and extrinsic autonomic nerves.

Question 19

What does the Gastrocolic reflex initiate?

Answer 19

Mass movements in the colon.

Question 20

The defecation reflex occurs in response to what?

Answer 20

Distention of the rectal wall.

Question 21

What are the 6 key points of the defecation reflex?

Answer 21

1. Contraction of the rectum
2. Relaxation of the internal anal sphincter
3. An initial contraction of the external anal sphincter
4. Increased peristaltic activity in the sigmoid colon
5. Relaxation of the external anal sphincter
6. Expulsion of faeces

Question 22

Voluntary defecation involves which manoeuvre?

Answer 22

Valsalva manoeuvre

Question 23

In relation to voluntary defecation, explain the 4 steps of the Valsalva manoeuvre.

Answer 23

1. Full inspiration followed by forced expiration against a closed glottis causes the diaphragm to move downwards.
2. The abdominal and thoracic muscles are contracted.
3. Increased pressure in the abdomen forces faecal contents into the rectum.
4. The defecation reflex is initiated.

Question 24

Describe the composition of faeces.

Answer 24

3/4 water
1/4 solid matter:

30% dead bacteria 
10-20% fat
10-20% inorganic matter
2-3% protein
30% undigested material, including bile pigments and sloughed epithelial cells.

Question 25

List some functions of intestinal bacteria.

Answer 25

1. Provides immunity against common bacteria
2. Converts bilirubin to urobilinogens
3. Degrades digestive enzymes
4. Digests mucus
5. Metabolizes undigested polysaccharides