The structures involved in maintaining continence and defaecation

The structures involved are the rectum, the internal anal sphincter (IAS) and the external anal sphincter (EAS). These are illustrated in the figure below.

Anal sphincters

Figure 4.2 Anal sphincters.

The anal sphincters are ring-like structures surrounding the anal canal.

Did you know? The anal sphincter mechanism is complex. With normal anorectal sensation the individual can identify the consistency of the stool, if it is solid, liquid or gas and even in an upright position, the sphincter can release gas while retaining liquid/solid.

Key facts about the rectum and anal sphincters

Here are some key facts that will help you to understand how the rectum and anal sphincters maintain continence and defaecation.

Rectum

  • The rectum functions as a reservoir for the stool until defaecation
  • It starts at the sigmoid colon and is 12-15cm in length
  • Hollow structure, holds 5-10ml at rest, dilates with a capacity of 300-500ml
  • Compliance – rectum is low pressure storage
  • Mucosa is made of goblet epithelial cells and secretes mucous
  • Sensation/awareness of stool in the rectum - stretch receptors in the muscle layers and pelvic floor, sampling of contents allow subconscious perception of the consistency of the contents
  • The sigmoid-rectal junction controls the passage of faecal matter from the sigmoid colon If the rectum is empty there is no desire to defecate
  • If the stool remains in the rectum, as a result of the individual not responding to the desire to defecate, the individual will lose the feeling they need to defecate and this can lead to constipation and/or faecal incontinence

Anal sphincters

Internal anal sphincter (IAS) External Anal sphincter (EAS)
Smooth muscle Striated muscle innervated by the pudendal nerve
Involuntary control Voluntary control – can exercise and strengthen the sphincter - anal sphincter exercises
The IAS maintains at 85% resting tone – this prevents faecal leakage The EAS contributes 15% of the resting anal tone
Pressure greater at lower end – when doing a rectal examination it is tighter at the bottom where the pressure is greatest Research suggests that when ‘straining’ you get high activity of EAS – strain tightens the sphincter. The external sphincter exerts it influence lower down than the internal sphincter
Reflex response of IAS is relaxation When stool is present in the rectum there is an increase in activity of the EAS and the EAS tightens
Defect of IAS can cause passive soiling Defect of the EAS can cause bowel urgency
  Fatigues when gets tired

Anal sphincters

Figure 4.3: Anal sphincters and mucosa.

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

Association for Continence Advice

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