Anatomy and physiology of anorectum: the hypothesis of fecal retention, and defecation

Abstract

Continence and defecation are two essential functions of the human body. Continence is the ability to retain feces until an acceptable time for defecation. Defecation is the evacuation of fecal material from the colon. Both functions involve complex physiologic processes that are not completely understood. The normal anatomy of the gastrointestinal tract is well documented in the literature. The data on the physiology of these processes are controversial and sketchy. For example, muscular contraction is accompanied by energy expenditure and resource depletion. All the known muscles, including the heart, relax after the contraction. During the relaxation, their contractile capacity is recovered. It is still not clear how the anal canal is in the continuous contraction, and which muscles are involved in the act of defecation. It is known that the external anal sphincter (EAS) is responsible for the emergency continence during the increase of abdominal pressure. However, the intensity of contraction promptly decreases after 8-15 seconds. It is hard to ascribe the continence to continuous contraction of the internal anal sphincter (IAS). A clear understanding of the normal physiology of the anorectal zone could significantly improve our approach to such pathological conditions like chronic constipation, fecal incontinence, anorectal malformations, etc. This work presents a new hypothesis of the continence and defecation, based on analysis of the literature and our own studies.