the physiology of defecation

Question 1

functions of the colon (3)

Answer 1

• absorption of water and electrolytes (osmosis)
• excretion of waste (motility)
• production of vitamins (microbiome)

Question 2

layers of the colonic wall (inner to outermost)

Answer 2

1. mucosa
2. muscularis mucosae
3. submucosa
4. muscularis propia
5. subserosa
6. serosa

Question 3

describe the muscular layers of the colon

Answer 3

• continuous circular muscle
• 3 ‘ribbons’ of longitudinal muscle - taeniae coli

Question 4

main muscular layer

Answer 4

muscularis propia - generates high amplitude contractions

Question 5

outer muscular layer

Answer 5

longitudinal muscle - formed as 3 ribbons/ strands

Question 6

histology of the colon

Answer 6

• simple columnar epithelium
• goblet cells

Question 7

nerve supply of colon and rectum

Answer 7

intrinsic and extrinsic

Question 8

what is intrinsic nervous system made up of

Answer 8

1. myenteric plexus
2. submucosal plexus

these are dense networks of neurones

Question 9

what is extrinsic nervous system made of

Answer 9

1. parasympathetic
2. sympathetic

Question 10

what is the anal sphincter

Answer 10

a group of muscles at the end of the rectum that surrounds the anus and controls the release of stool, thereby maintaining continence

Question 11

what are the 2 muscles of the anal sphincter

Answer 11

1. internal anal sphincter
2. external anal sphincter

Question 12

4 phases of defecation

Answer 12

1. basal
2. pre - expulsive
3. expulsive
4. termination

Question 13

function of rectum

Answer 13

temporary storage of faeces before defecation

Question 14

what causes the desire to defecate

Answer 14

• as the rectum becomes filled the rectal walls expand and stretch receptors stimulate the desire to defecate.
• the urge to defecate arises from:
  • contraction of rectal muscles,
  • relaxation of the internal anal sphincter
    -and an initial contraction of the external anal sphincter

Question 15

what happens if the urge to defecate is not acted upon

Answer 15

further water is absorbed and the faeces is stored until the next mass movement occurs

Question 16

what is the internal anal sphincter controlled by

Answer 16

parasympathetic fibres which relax involuntarily

Question 17

what is the external anal sphincter controlled by

Answer 17

it is s skeletal muscle

is controlled by somatic nerve supply from the Inferior anal branch of the Pudendal nerve (S2,3,4)

which allows conscious control of defecation

Question 18

what happens when the rectum is distended

Answer 18

the rectosphincteric reflex is initiated and relaxes the internal sphincter

Question 19

what happens if defecation is not desired

Answer 19

voluntary contraction of the external sphincter can delay it

Question 20

what happens if defecation is appropoiate

Answer 20

a series of reflexes take place that lead to:

• Relaxation of the external sphincter
• Contraction of abdominal wall muscles
• Relaxation of pelvic wall muscles

Question 21

what do peristaltic waves do

Answer 21

facilitate the movement of faeces through the anal canal

Question 22

what is the Valsalva maneuver

Answer 22

when defecation is assisted by taking a deep breath and attempting to expel the air against a closed glottis

Question 23

what happens if a delay in defection is needed

Answer 23

voluntary contraction of the external sphincter is usually sufficient to override the reflexes that anal distension initiates

Question 24

what occurs in the colon during the basal phase of defecation

Answer 24

segmental contractions (mixing)

Question 25

what occurs in the rectum during the basal phase of defecation

Answer 25

motor complexes keep it empty

Question 26

what occurs in the anal sphincter during the basal phase of defecation

Answer 26

tonic contraction

Question 27

what happens to puborectalis muscle during basal phase of defecation

Answer 27

it is contracted

at a 90 degree anorectal angle

Question 28

what occurs in the colon during the pre expulsive phase of defecation

Answer 28

there are high amplitude propagating contractions

mass movement of stool

gastro colic reflex

Question 29

what occurs in the rectum during the pre expulsive phase of defecation

Answer 29

it fills causing distension

rectal compliance ( adaptive relaxation)

Question 30

what occurs in the anal sphincter during the pre expulsive phase of defecation

Answer 30

the external anal sphincter maintains contraction

reflex relaxation of internal anal sphincter (RAIR) - for stool sampling

Question 31

what happens to puborectalis muscle during the pre expulsive phase of defecation

Answer 31

it remains contracted

Question 32

what happens in the explusive phase of defecation

Answer 32

• rectum contracts
• internal & external anal sphincter and puborectalis relaxes
• valsalva manoeuvre / posture aid emptying

Question 33

what happens in sitting posture

Answer 33

the puborectalis muscle chokes the rectum to maintain continence

Question 34

what happens in squatting posture

Answer 34

the puborectalis muscle relaxes and straightens the pathway to the anus

Question 35

what happens in the termination phase of defecation

Answer 35

• traction loss causes sudden contraction of external anal sphincter (“closing reflex”)
• valsava closes
• change in posture (to standing)

Question 36

2 disorders of defecation

Answer 36

1. constipation
2. diarrohea

Question 37

what are the things that go wrong to cause constipation

Answer 37

• consistency of stool
• bowel motility
• physical blockage to the bowel
• pelvic floor disorders

Question 38

what are the disorders that go wrong to cause diarrhoea

Answer 38

• consistency of stool
• frequency of movements
• diseased bowel mucosa
• reduced rectal capacity
• pelvic floor disorder

Question 39

causes of faecal incontinence

Answer 39

• Physical injury
• Nerve injury
• Surgery
• Intense fright
• Inflammatory bowel disease
• Childbirth