Physiology of defaecation

In this step we explore the physiology of defaecation including the anal sphincter mechanism, the defaecation process and the implications on both of not having the correct posture on the toilet.

Control of defaecation

The basic requirements for bowel continence are:

• Rectum – a compliant reservoir with adequate capacity
• Sphincters – intact internal anal sphincter (IAS) and external anal sphincter (EAS) which generate adequate pressures
• Normal sensation – rectal distension, anal sensation and reflex contractions
• Co-ordinating system – the brain and nervous systems

Anal sphincter mechanism

In the previous step we looked at the anatomy of the bowel and the anal sphincters and now we look at the anal sphincter mechanism in greater detail and the balance between maintaining continence and expelling a stool.^{Figure 4.4: Anal sphincter mechanism when the rectum is empty.}

• When the rectum is empty:
  • Contraction of puborectalis muscle produces anorectal angulation
  • Internal anal sphincter (IAS) contracts
  • External anal sphincter (EAS) contracts
  • Continence is maintained

Note: For continence to be maintained the anorectal angle must be less than 90 degrees.^{Figure 4.5: Anal sphincter mechanism during defaecation.}

• During defaecation:
  • Sensation of stool in rectum
  • IAS relaxes
  • EAS relaxes
  • Puborectalis muscle relaxes
  • Rectum contracts
  • Abdominal muscles contract
  • Stool is expelled

Defaecation process

The defaecation process is:

• Triggered by gastro-colic reflex (this will be covered in greater detail later on in the week).
• Stool and gas transported to rectum – peristaltic contractions
• Usually initiated by a meal
• Distention of rectum – registered in cerebral cortex
  • Reflex relaxation of IAS
  • Stool moves down coming in contact with receptors in upper anal tract
  • Sampling reflex

For defaecation to occur :

• Further contraction of rectal muscles
• Relaxation of EAS
• Puborectalis muscle relaxes, widening the anorectal angle and opening the anal canal

Correct posture on the toilet

Clinical practice point. The correct posture on the toilet is essential to relax the puborectalis muscle, widen the angle and open the canal.

^{Figure 4.6 Correct posture on the toilet to relax the puborectalis muscle and straighten pathway to anus}