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Understanding the physiology of human defaecation and disorders of continence and evacuation

Abstract

The act of defaecation, although a ubiquitous human experience, requires the coordinated actions of the anorectum and colon, pelvic floor musculature, and the enteric, peripheral and central nervous systems. Defaecation is best appreciated through the description of four phases, which are, temporally and physiologically, reasonably discrete. However, given the complexity of this process, it is unsurprising that disorders of defaecation are both common and problematic; almost everyone will experience constipation at some time in their life and many will develop faecal incontinence. A detailed understanding of the normal physiology of defaecation and continence is critical to inform management of disorders of defaecation. During the past decade, there have been major advances in the investigative tools used to assess colonic and anorectal function. This Review details the current understanding of defaecation and continence. This includes an overview of the relevant anatomy and physiology, a description of the four phases of defaecation, and factors influencing defaecation (demographics, stool frequency/consistency, psychobehavioural factors, posture, circadian rhythm, dietary intake and medications). A summary of the known pathophysiology of defaecation disorders including constipation, faecal incontinence and irritable bowel syndrome is also included, as well as considerations for further research in this field.

Key points

  • Defaecation is a fundamental physiological process resulting in the evacuation of faeces; it is dependent on the coordination of neural, muscular, hormonal and cognitive systems.

  • Several factors influence defaecation, including gastrointestinal transit, stool volume and/or consistency, and dietary intake.

  • Defaecation can be described in terms of four reasonably discrete temporal phases: basal phase, pre-expulsive phase, expulsive phase and end phase.

  • The latest imaging and technological advances (such as high-resolution colonic and anorectal manometry, cine-MRI and magnetic resonance defaecography and wireless capsules) have improved our knowledge of defaecatory mechanisms.

  • Knowledge of the physiology of normal defaecation could inform management of common disorders of defaecation such as constipation and faecal incontinence; however, future research needs are highlighted in this article.

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Fig. 1: Neuromuscular anatomy of the colon and anorectum.
Fig. 2: Principal events before and during defaecation.
Fig. 3: High-resolution colonic manometry recordings in four healthy volunteers.
Fig. 4: Anorectal opening mechanism during defaecation.
Fig. 5: Barium (X-ray) defaecography images.

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Acknowledgements

P.T.H. is the recipient of the Peter King Research Scholarship, Royal Australasian College of Surgeons.

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Heitmann, P.T., Vollebregt, P.F., Knowles, C.H. et al. Understanding the physiology of human defaecation and disorders of continence and evacuation. Nat Rev Gastroenterol Hepatol 18, 751–769 (2021). https://doi.org/10.1038/s41575-021-00487-5

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