Physiology Of Defaecation

Question 1

What are some functions of the colon?

Answer 1

Absorption of water and electrolytes
Production of vitamins
Excretion of waste

Question 2

What are the layers of the colonic wall?

Answer 2

Mucosa
Muscularis mucosae
Submucosa
Muscularis propia
Subserosa
Serosa

Question 3

What are the muscular layers of the colon?

Answer 3

• Continuous circular muscle
• 3 “stripes” of longitudinal muscle – taeniae coli

Question 4

What type of cells does the colon have?

Answer 4

• Columnar epithelium
• Goblet cells

Question 5

How are water and electrolytes absorbed through the colon?

Answer 5

• Water is absorbed by osmosis
• Sodium is actively transported

Question 6

What is the intrinsic nerve supply to the colon?

Answer 6

Meissners and Auerbach’s plexus

Question 7

What is the extrinsic nerve supply to the colon?

Answer 7

Parasympathetic
Sympathetic

Question 8

What is the gastro-colic reflex?

Answer 8

Stomach stretching and food in the jejunum leading to mass movement of the colon

Question 9

What happens to the sphincters and puborectalis muscle when the rectum is empty?

Answer 9

Both sphincters are contracted
Puborectalis muscle is contracted

Question 10

How do we know we need a poo?

Answer 10

Rectum fills
Reflex relaxation of internal anal sphincter
Sampling reflex

Question 11

What happens to the body for defaecation to be completed?

Answer 11

External sphincter relaxes
Puborectalis relaxes
Rectum contracts
Valsalva maneuver

Question 12

What happens to the puborectalis muscle during excretion whilst having a sitting posture?

Answer 12

Puborectalis muscle ‘chokes’ the rectum to maintain continence

Question 13

What happens to the puborectalis muscle during excretion whilst having a squatting posture?

Answer 13

Puborectalis muscle relaxes and straightens pathway to anus

Question 14

What are some disorders of defaecation?

Answer 14

Consistency of stool
Bowel motility
Physical blockage to the bowel
Pelvic floor disorders

Question 15

What occurs during diarrhoea?

Answer 15

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

Question 16

What are the 3 parts the colon can be split into?

Answer 16

Ascending colon
Transverse colon
Descending colon

Question 17

What is the continuous circular muscle important for?

Answer 17

Mass movement

Question 18

What nerve innervates the ascending and transverse colon?

Answer 18

Vagus nerve

Question 19

What nerve innervates the descending colon?

Answer 19

Pelvic nerve

Question 20

What is the final part of the digestive tract?

Answer 20

Anal sphincter

Question 21

What muscles are found in the anal sphincter?

Answer 21

Internal anal sphincter muscle
- smooth muscle wall
- involuntary muscle
- parasympathetic pelvic nerve

External anal sphincter muscle
- voluntary
- striated muscle
- somatic pudendal nerve

Question 22

What are the 4 phases of defecation?

Answer 22

Basal
Pre-expulsive
Expulsive
Termination

Question 23

What occurs in the basal phase?

Answer 23

Colon – segmental contractions (mixing)
Rectum - motor complexes (to keep rectum empty) “braking mechanism”
Anal Sphincter - tonic contraction
Puborectalis - contracted (90o anorectal angle)

Question 24

What happens in the pre-expulsive phase?

Answer 24

Colon – high amplitude propagating contractions
- Mass movement of stool ~8 times day
- Gastro-colic reflex

Rectum
- Fills causing distension
- Rectal compliance (adaptive relaxation)

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

Puborectalis – remains contracted

Question 25

What happens in the expulsive phase?

Answer 25

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

Question 26

What happens in the termination phase?

Answer 26

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

Question 27

What factors can cause constipation?

Answer 27

Diet - not enough fibre
Lack of fluid intake
Medication - Opioids (Codeine)
Under-active thyroid
Hypercholemia
Diabetes
Parkinsons
Tumours
Scar tissue