The rectum is the final segment of the large intestine, a muscular tube linking the sigmoid colon to the anal canal and anus. Located in the pelvic area, it serves as the endpoint of the digestive process where solid waste accumulates. The primary purpose of the rectum is to manage the temporary containment and controlled elimination of feces from the body.
Temporary Storage of Waste
The rectum acts as a temporary reservoir for fecal matter before elimination. Unlike the colon, which absorbs water and electrolytes, the rectum is structured for compliant storage. The final section, the rectal ampulla, is designed to relax and stretch as waste enters from the sigmoid colon.
This capacity to expand, known as compliance, allows the rectum to hold a significant volume of stool without triggering an immediate urge to defecate. The rectal wall contains folds that help support the weight of accumulating feces. This storage ability prevents the continuous, small-volume passage of stool that would occur if the rectum acted merely as a conduit.
As the rectal ampulla slowly fills, its muscular walls adapt to the increasing volume, ensuring a pressure gradient that favors storage over expulsion. If waste were to remain in the colon, excessive water absorption would occur, leading to harder, more difficult-to-pass stool. The rectum’s storage function therefore contributes both to continence and to maintaining an appropriate consistency for elimination.
The Defecation Reflex
Elimination begins with the defecation reflex, a coordinated sequence of neural signals triggered by the mechanical distension of the rectal wall. As stool enters, sensory receptors embedded in the muscular tissue are activated by the stretch. These mechanoreceptors transmit signals via the pelvic nerves to the sacral segments of the spinal cord.
This sensory input initiates an involuntary reflex response within the nervous system. The immediate effect is the relaxation of the internal anal sphincter, a ring of smooth muscle that is normally contracted to maintain closure. Simultaneously, the parasympathetic nerves trigger strong peristaltic contractions in the rectum and sigmoid colon, pushing the fecal matter downward toward the anal canal.
The relaxation of the internal sphincter allows a small amount of rectal content to move into the upper anal canal, which provides the sensation necessary to distinguish between gas, liquid, or solid stool. At this stage, the signal also travels to the cerebral cortex, creating the conscious awareness and urge to defecate. If the time is inappropriate for elimination, the reflex can be voluntarily suppressed.
Voluntary suppression involves the conscious contraction of the external anal sphincter and the puborectalis muscle. If elimination is appropriate, the voluntary relaxation of these skeletal muscles, combined with increased intra-abdominal pressure from straining, straightens the anorectal angle. This coordinated action allows the propulsive waves of the rectum to effectively expel the waste.
Maintaining Continence
The rectum works in concert with the muscles of the pelvic floor to maintain continence. This control is achieved through two distinct muscular rings that guard the anal canal. The internal anal sphincter, composed of smooth muscle, provides the majority of the resting anal tone, acting as a constant, involuntary barrier against leakage.
The external anal sphincter is a ring of voluntary skeletal muscle, consciously tightened to reinforce the barrier during moments of increased abdominal pressure, such as coughing or lifting. This contraction is supported by the puborectalis muscle, a sling-like structure looping around the anorectal junction. The puborectalis muscle maintains a resting angle between the rectum and the anal canal.
This angulation creates a physical kink that prevents the unintentional passage of stool, acting as a flap-valve mechanism. Damage to the nerve supply or the integrity of these muscular structures can significantly compromise the ability to maintain continence. The involuntary contraction of the puborectalis muscle also provides an immediate, reflex-based barrier against sudden increases in intrarectal pressure.
Common Functional Rectal Conditions
Disruption to the complex functional coordination of the rectum can lead to several common clinical conditions. Fecal incontinence, for instance, occurs when there is a breakdown in the storage, sensation, or sphincter control mechanisms. This can result from nerve injury, muscle damage to the sphincters, or a loss of rectal wall compliance.
Chronic constipation often involves issues with the defecation process, sometimes linked to a condition called non-relaxing puborectalis syndrome. In this case, the puborectalis muscle fails to relax properly during attempted elimination, which prevents the anorectal angle from straightening and obstructs the passage of stool. Rectal prolapse, a structural failure, occurs when the rectum telescopes and protrudes through the anus.
This structural displacement impairs both reservoir function and continence mechanisms. Conditions like proctitis, inflammation of the rectal lining, can disrupt sensory function by making the rectum hypersensitive to small volumes of stool. This leads to a frequent, urgent, and often painful urge to defecate, even when the rectum is not full.