The Physiology Of Defacation

Question 1

What are the functions of the colon?

Answer 1

• Absorption of water and electrolytes (osmosis)
• Excretion of waste (motility)
• Production of vitamins (micro biome)

Question 2

What are the 6 layers of the colonic wall?

Answer 2

Starting inside - out:
- Mucosa
- Muscularis mucosae
- Submucosa
- Muscularis propria (main muscle layer)
- Subserosa
- Serosa

Question 3

What types of muscle are in the muscle layers?

Answer 3

• Continuous circular muscle
• 3 ribbons of longitudinal muscle - taeniae coli
• make high amplitude movements

Question 4

What epithelium does the colon have?

Answer 4

Simple columnar epithelium

Question 5

Does the colon have goblet cells?

Answer 5

Yes - produces mucus

Question 6

What is the nervous system for the intestinal wall?

Answer 6

Enteric nervous system

Question 7

What is the intrinsic nerve supply?

Answer 7

• Myenteric plexus
• Submucosal plexus

Question 8

What is the extrinsic nerve supply?

Answer 8

• parasympathetic (vagus)
• sympathetic

Question 9

What are the 2 anal sphincters?

Answer 9

• Internal anal sphincter (involuntary + parasympathetic)
• External anal sphincter (voluntary as striated muscle)
  External would be patient stopping them self from going if not in appropriate place

Question 10

What are the 4 phases of defecation?

Answer 10

1. Basal
2. Pre-expulsive
3. Expulsive
4. Termination

Question 11

Why happens in the basal phase of defecation?

Answer 11

• Colon has segmental contractions (mixing)
• Rectum has motor complexes (braking mechanism) to keep the rectum empty
  -Anal sphincter undergoes tonic contraction (the sustained contraction of different groups of fibers within a muscle to maintain continual muscular tension)
• the puborectalis contracted to a 90 degree angle

Question 12

What is the anorectal angle?

Answer 12

The puborectalis muscle forms a loop around the posterior aspect of the external sphincter. Contraction of the puborectalis muscle creates an anorectal angle.

Question 13

What happens in the pre-expulsive phase?

Answer 13

Colon: high amplitude propagating contractions
- Mass movement of stool
- Gastro-colic reflex
Rectum:
- Fills causing distension
- Rectal compliance (adaptive relaxation)
- Puborectalis – remains contracted

Question 14

What happens to the anal sphincter during the pre-expulsive phase?

Answer 14

Anal Sphincter:
- EAS maintains contraction
- Reflex relaxation of IAS (RAIR) – for stool sampling

Question 15

What happens in the expulsive phase?

Answer 15

• Rectum contracts
• IAS, EAS and PR relaxes
• Valsalva manoeuvre/posture aid emptying (squatting)

Question 16

Why is the squatting/ valsalva position preferred?

Answer 16

Puborectalis muscle relaxes and straightens pathway to anus

Question 17

What happens in the termination phase?

Answer 17

• Traction loss causes sudden contraction of EAS (“closing reflex”)
• Valsalva ceases
• Change in posture (to standing)

Question 18

What happens when someone has constipation?

Answer 18

• Consistency of stool hardens
• Bowel motility (nerves affect movement)
• Physical blockage to the bowel
• Pelvic floor disorders

Question 19

What happens when someone has diarrhoea?

Answer 19

• Consistency of stool or frequency of movements
• Diseased bowel mucosa
• Reduced rectal capacity
• Pelvic floor disorder