The rectum is the final segment of the large intestine, a muscular tube extending from the sigmoid colon to the anal canal. Measuring approximately 12 to 15 centimeters in length, it is situated deep within the pelvis. This portion of the digestive tract plays a specific and highly specialized role in the elimination process. Its primary functions are centered on managing fecal waste, including temporary storage and the precise control required for both continence and defecation. The rectum acts as a critical intermediary, bridging the digestive system’s waste processing with the body’s ability to excrete material.
Role as a Temporary Storage Reservoir
The rectum’s main purpose is to serve as a holding area for fecal matter before it is expelled from the body. This storage capability is necessary for regulating bowel movements and allowing the deferral of defecation until a socially acceptable moment. The final section of the rectum, known as the rectal ampulla, is the specific reservoir that accommodates the waste.
By the time waste reaches the rectum, most water and electrolytes have been absorbed, resulting in a semi-solid consistency. As the rectal ampulla fills, its walls relax and stretch to accommodate the increasing volume, a property known as rectal compliance. This compliance means the rectum can often tolerate volumes exceeding 300 milliliters before the feeling of urgency becomes intense.
The Mechanics of Defecation
The process of elimination is initiated when mass movements from the colon propel fecal material into the empty rectum, causing distension. This stretching of the rectal wall stimulates mechanoreceptors, which send afferent signals to the central nervous system, triggering the conscious urge to defecate. This sensory input begins the involuntary neurological process known as the defecation reflex.
The defecation reflex involves the involuntary relaxation of the internal anal sphincter (IAS), a ring of smooth muscle, while the smooth muscle of the sigmoid colon and rectum contracts more vigorously. This relaxation of the IAS is termed the rectoanal inhibitory reflex, allowing a small amount of material to move into the anal canal for “sampling” by specialized sensory mucosa. If the circumstances are suitable for elimination, the voluntary muscles, specifically the external anal sphincter (EAS) and the puborectalis muscle, relax to straighten the anorectal angle. The final expulsion is often assisted by voluntary straining, which involves contracting the abdominal muscles to increase intra-abdominal pressure and force the feces out.
Maintaining Continence
Continence, or the ability to prevent involuntary leakage of feces or gas, is a coordinated function of the rectum, pelvic floor muscles, and the anal sphincters. The internal anal sphincter, composed of involuntary smooth muscle, is responsible for the majority of the resting pressure in the anal canal, contributing approximately 50 to 85 percent of the basal tone. This constant contraction is maintained by the autonomic nervous system and prevents the passive leakage of liquid and gas.
The external anal sphincter (EAS) is made of striated muscle and is under conscious, voluntary control, providing a high-pressure zone to prevent leakage when the internal sphincter relaxes. When the rectum is distended and the internal sphincter relaxes, the EAS contracts reflexively and voluntarily to maintain the barrier. Additionally, the puborectalis muscle forms a U-shaped sling around the anorectal junction, maintaining a sharp bend known as the anorectal angle. This angle acts as a mechanical barrier, assuring control until the moment of intentional defecation.