Anal Physiology: The Physiology of Continence and Defecation

  • Vitaliy Y. Poylin
  • Thomas E. CataldoEmail author


The physiology of the anus and its surrounding structures is the physiology of continence and controlled defecation. This is a physiology of balance and continuous feedback and complex reflexes. Continence requires balance between the pressure inside the rectum and the combined tone of the internal and external sphincters. This chapter provides an overview of the anatomy, specifically the innervation of the anal complex with regard to how it affects normal and pathologic defecation. The neurosensory-motor aspects of the sequences of events of defecation are reviewed. Conditions that result in disordered defecation, pelvic pain, and complications outside the GI system are discussed.


Physiology Anorectal innervation Defecation Continence Pelvic floor RAIR Anorectal pain Obstructed defecation 



Fecal incontinence


Magnetic resonance


Rectoanal inhibitory reflex


Sacral nerve stimulation

Supplementary material

Video 3.1

Normal defecography. (Courtesy of Shauna Lorenzo-Rivero, MD, FACS, FASCRS) (MP4 17418 kb)

Video 3.2

Defecography of a patient with a rectocele. (Shauna Lorenzo-Rivero, MD, FACS, FASCRS) (MP4 10698 kb)

Copyright information

© ASCRS (American Society of Colon and Rectal Surgeons) 2019

Authors and Affiliations

  1. 1.Division of Colon and Rectal Surgery, Department of Surgery, Beth Israel Deaconess Medical CenterHarvard Medical SchoolBostonUSA

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