The normal function of the anorectum is storage and appropriate release of intestinal waste products. The rectum functions mainly as a capacitance storage vessel. The normal rectum holds 650-1200 mL of waste. Resting rectal pressure is approximately 10 mm Hg. Changes in intrarectal pressure are primarily a reflection of intraabdominal pressure changes, as the rectum itself has little peristaltic function.
The function of the pelvic floor is complex and poorly understood. The levator ani create a broad muscular funnel suspending the rectum in a muscular sling that pulls the rectum forward at the anorectal junction creating an acute anorectal angle. The acuity of the angle created by the puborectalis is critical for maintaining continence. The puborectalis contracts (increasing the anorectal angle) during maneuvers that increase intraabdominal pressure (such as coughing, laughing), maintaining continence, but relaxes, opening the angle, when a Valsalva maneuver is performed as part of normal defecation. The levators also contain sensory fibers that detect pelvic fullness and therefore are believed to be important in the sensation of the urge to defecate.
The internal sphincter generates 85% of the resting anal sphincter tone. It is innervated with sympathetic and parasympathetic fibers that are both inhibitory and keep the sphincter in a constant state of contraction. The external sphincters are skeletal muscles innervated by the pudendal nerve with fibers that originate from S2-4.
The external sphincter provides 15% of the resting anal sphincter tone and 100% of the voluntary squeeze pressures. Voluntary contraction of the external sphincter can double the resting anal sphincter pressure but voluntary contraction cannot be sustained for more than 3 minutes.
Hemorrhoids are important participants in maintaining continence and minimizing trauma during defecation. They function as protective pillows that engorge with blood during the act of defecation molding the stool and protecting the anoderm from direct trauma when stool passes. They also act as cushions that help seal the anal canal and prevent leakage of gas and stool. Hemorrhoidal tissues become engorged during defecation and lifting, and with other conditions that increase intraabdominal pressure such as obesity and pregnancy.
Defecation is a complex event involving multiple steps for successful evacuation. The anal sphincters function in concert with the levator ani, the puborectalis, and the rectum to promote defecation at socially acceptable times. Continence is maintained so long as intrarectal pressures are lower than internal and external sphincter pressures. In the resting state the rectum is not completely empty, however residual contents are not sensed. The rectum relaxes (accommodates) and sensory fibers in the levators adapt to the sense of pelvic fullness allowing these contents to remain. Periodically, the internal sphincter relaxes, allowing the rectal contents to drop down into the anal canal where they are sampled by the sensitive anoderm. After sampling, the external sphincter contracts and the contents are pushed back into the rectum. This “sampling reflex” or rectoanal inhibitory reflex also occurs up to seven times daily during periods of rectal distention. Progressive distention of the rectum eventually causes continuous inhibition of the internal sphincter and relaxation of the external sphincter causing an urge to defecate. If one wishes to evacuate, a sitting/squatting position is assumed (straightening the anorectal angle), intraabdominal pressure is increased by a Valsalva maneuver, the puborectalis relaxes (further shortening the anal canal and promoting formation of a muscular funnel), and reflex relaxation of the internal sphincter occurs as fecal contents enter the anal canal. The Valsalva maneuver primarily accomplishes evacuation.
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Source: Mark Lane Welton, MD
Portions of this article are reprinted from Way LW, Doherty GM (eds): Current Surgical Diagnosis & Treatment, 11th ed. New York: McGraw-Hill, 2003