Large Intestine Structure & Function

Question 1

What is the structure of the large intestine?

Answer 1

Large components:

Caecum

Ascending colon

Transverse colon

Desecending colon

Sigmoid colon

Musculature:

3 bands of tenia coli running length of colon (contraction produces haustra)

Circular muscle layer complete, longitudinal muscle layer incomplete

Question 2

What is the basic histology of the large intestine?

Answer 2

• Simple columnar epithelium
• Large straight crypts lined with large number of goblet cells (provide lubrication for faeces movement)

Question 3

What is the importance of the intestinal bacteria in the digestive process?

Answer 3

• Microbiota of the intestine metabolise dietary components
• Gut microbiota breakdown dietary fibre and convert it into thousands of different products.
• Provide defence against pathogens

Question 4

What is the function of the large intestine?

Answer 4

Absorbs water and ions to form faeces

Question 5

What is the structure of the rectum, anal canal and rectal sphincters?

Answer 5

Rectum – Straight muscular tube between end of sigmoid colon and anal canal

Anal canal – 2-3cm between distal rectum and anus. The muscularis is thicker than rectum. The external anal sphincter is made up of skeletal muscle.

Anal sphincters:

• Internal anal sphincter is involuntary muscle that surrounds the upper 2/3rds of the anal canal. It is formed from a thickening of the smooth muscle in the bowel wall.
• External anal sphincter is a voluntary muscle that surrounds the lower 2/3 of the anal canal (therefore it crosses over with the internal anal sphincter.

Question 6

What are the mechanisms controlling defacation?

Answer 6

Stretching of the rectum by arrival of bolus causes mechanoreceptors to fire triggering the defacation reflex

Defacation reflex:

1) Contraction of rectum
2) Relaxation of internal sphincter and contraction of external anal sphincters
3) Increased peristaltic activity in the colon increases the pressure on the external sphincter which eventually relaxes under voluntary control.
4) The defacation reflexes are under the control of the parasympathetic pelvic splanchnic nerves.

Question 7

What are the mechanisms that cause secretory diarrhoea?

Answer 7

Secretory diarrhoea can be caused by:

• Bacteria which produce the protein enterotoxins
• Protein enterotoxins turn on intestinal chloride secretion from crypt cells, which increases the water secretion into the intestine
• The enterotoxins work by elevating intracellular messengers:
• cAMP
• cGMP
• Calcium
• The excess water secretions are too much for the villi to absorb, so this leads to profuse watery diarrhoea.

Question 8

How can diarrhoea be treated with simple salt/sugar solutions?

Answer 8

Sodium/glucose solution will drive the absorption of the excess water and rehydrate the patient