Defecation - Podcast Version 0:00 / 0:00 1x 0.25x 0.5x 0.75x 1x 1.25x 1.5x 1.75x 2x Defecation is the process by which faeces are expelled from the gastrointestinal tract (GI tract) through the anus. It is a complex physiological process requiring co-ordinated activity of the gastrointestinal, nervous and musculoskeletal systems. It involves both involuntary and voluntary control. This article will describe the mechanisms involved in defecation and consider relevant clinical conditions associated with dysfunction. Pro Feature - 3D Model You've Discovered a Pro Feature Access our 3D Model Library Explore, cut, dissect, annotate and manipulate our 3D models to visualise anatomy in a dynamic, interactive way. Learn More Rectal Filling Faeces are propelled into the rectum by mass movements of the colon. These are powerful, propagating contractions that occur several times per day and are often stimulated by the gastrocolic reflex, whereby gastric distension following a meal increases colonic motility. As faeces enter the rectum, the rectal walls become distended. Stretch receptors within the rectal wall detect this distension, triggering local enteric reflexes and afferent signalling to the sacral spinal cord. These pathways produce the sensation of rectal fullness and the urge to defecate, initiating the defecation reflex. The rectum can store up to 300mL of faeces before the urge to defecate is perceived. 2015-2017 TeachMeAnatomy.com [CC-BY-NC-ND 4.0] Fig 1Sagittal section of the female pelvis, showing the anatomical position of the rectum. Defecation Reflex Rectal distension activates the defecation reflex – an involuntary reflex consisting of a weak intrinsic enteric component, reinforced by a parasympathetic component. The result is peristalsis of the rectal wall and relaxation of the internal anal sphincter. The intrinsic component is mediated by the myenteric plexus of the enteric nervous system. Rectal distension stimulates peristaltic contractions within the rectum and sigmoid colon, propelling faeces towards the anal canal. It also causes relaxation of the internal anal sphincter, known as the rectoanal inhibitory reflex (RAIR) or rectosphincteric reflex. In the parasympathetic component, stretch receptors within the rectal wall transmit afferent signals to the sacral spinal cord (S2-S4) via the pelvic splanchnic nerves. Parasympathetic efferent fibres then return to the distal colon and rectum, strengthening peristaltic contractions and promoting further relaxation of the internal anal sphincter. Afferent signals from the rectum also reach higher centres of the brain, allowing the individual to perceive the urge to defecate and voluntarily decide whether to proceed or defer defecation. By Boumphreyfr, via Wikimedia Commons (public domain) Fig 2Voluntary and parasympathetic pathways of the defecation reflex. Voluntary Control Conscious control of defecation involves the co-ordinated activity of the external anal sphincter and pelvic floor muscles, particularly the puborectalis muscle. External Anal Sphincter The external anal sphincter is composed of skeletal muscle and is under voluntary control. It is supplied by the inferior rectal nerve – a branch of the pudendal nerve (S2-S4). When defecation is appropriate, voluntary relaxation of the external anal sphincter allows faeces to pass through the anal canal. If defecation is not appropriate, the urge to defecate is suppressed by voluntary contraction of the external anal sphincter, allowing defecation to be delayed and contributing to faecal continence. The rectum accommodates to this continued distension, causing the sensation of urgency to diminish temporarily. Pelvic Floor The pelvic floor muscles contribute to both faecal continence and defecation. The puborectalis muscle forms a sling around the anorectal junction and maintains the anorectal angle, helping to prevent involuntary passage of stool. During defecation, the puborectalis and other pelvic floor muscles relax, allowing the anorectal angle to straighten and reducing resistance to the passage of faeces. Expulsion of Faeces On initiating defecation, the internal and external anal sphincters and pelvic floor muscles relax in a co-ordinated manner, allowing faeces to pass through the anal canal. Contraction of the abdominal wall muscles increases intra-abdominal pressure and assists defecation. During straining, a person may perform the valsalva manoeuvre by attempting to exhale against a closed glottis. This further increases intra-abdominal pressure and facilitates the expulsion of faeces. Clinical Relevance Faecal Incontinence Although rectal distension and relaxation of the internal anal sphincter are largely involuntary, voluntary control of the external anal sphincter allows defecation to be delayed until an appropriate time. Voluntary control of defecation is an important developmental landmark usually acquired by 3 years of age. Faecal incontinence is the involuntary leakage or passage of faecal material. It can result from disruption of the anal sphincters, pelvic floor, rectum or their associated sensory and motor pathways. Voluntary control can be temporarily or permanently be lost once it has been acquired (most common) or may never develop to begin with. Causes of faecal incontinence include: Anal sphincter injury Pudendal nerve or other neurological injury Pelvic floor dysfunction Colorectal or pelvic surgery Inflammatory bowel disease Childbirth-related trauma Acute gastroenteritis causing diarrhoea Constipation causing overflow diarrhoea Management depends on the underlying cause and may include dietary modification, treatment of diarrhoea or constipation, pelvic floor rehabilitation, medical therapy or surgical intervention. Clinical Relevance Constipation Constipation involves infrequent or difficult passage of stools, which may be hard, require excessive straining or cause a sensation of incomplete evacuation. It can result from reduced colonic motility, increased colonic water absorption, chronic suppression of the defecation reflex or dysfunction of the pelvic floor. Management depends on the underlying cause and may include dietary modification, optimising fluid intake or encouraging mobility. Laxatives (or aperients) are used in the pharmacological management of constipation and can be broadly divided into several classes: Laxative Type Examples Mechanism Bulk-Forming Ispaghula Husk Contain fibre that absorbs water and increases stool bulk, promoting intestinal motility. Stimulant Senna, Bisacodyl Increase colonic motility and fluid retention in the colon, accelerating faecal transit. Osmotic Lactulose, Macrogol Poorly absorbed from the GI tract, therefore increasing luminal osmolarity and drawing water into the bowel to soften stool. Stool-Softening Docusate Reduces surface tension, facilitating water penetration into faeces and softening the stool. When taking laxatives adequate water intake is important to allow them to work effectively and prevent dehydration. This is particularly important for bulk-forming and osmotic laxatives which draw water into the GI tract. Do you think you’re ready? Take the quiz below Pro Feature - Quiz Defecation Question 1 of 3 Submitting... Skip Next Rate question: You scored 0% Skipped: 0/3 More Questions Available Upgrade to TeachMePhysiology Pro Challenge yourself with over 2100 multiple-choice questions to reinforce learning Learn More Frequent questions What is the role of the rectum in defecation? The rectum serves as a temporary storage site for faeces before defecation. As it fills, stretch receptors trigger the urge to defecate, prompting muscular contractions and sphincter relaxation. How do the anal sphincters function during defecation? Defecation involves two anal sphincters: the internal sphincter, which relaxes involuntarily via parasympathetic control, and the external sphincter, which is under voluntary control through the pudendal nerve, allowing conscious regulation of the process. What physiological mechanisms facilitate the act of defecation? During defecation, the external sphincter relaxes, abdominal wall muscles contract, and pelvic wall muscles relax, while peristaltic waves propel faeces through the anal canal. The Valsalva maneuver can also assist in the process by increasing intra-abdominal pressure. What causes faecal incontinence? Faecal incontinence can result from various factors, including physical or nerve injury, surgery, intense fright, inflammatory bowel disease, and childbirth. Loss of control can occur if defecation happens too rapidly or if the body fails to absorb excess water. How do laxatives help treat constipation? Laxatives, such as bulk and stimulant types, aid in relieving constipation. Bulk laxatives increase intestinal mass and stimulate peristalsis, while stimulant laxatives enhance water and electrolyte retention in the gut, promoting bowel movements. Rate This Article