Physiology of defaecation
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.
- 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

- 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
- 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.

Understanding Continence Promotion: Effective Management of Bladder and Bowel Dysfunction in Adults

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