The rectum is the final segment of the large intestine, serving as a transitional area between waste processing in the colon and its final elimination. It is a muscular tube, approximately 12 to 15 centimeters (about 5 to 6 inches) long, connecting the S-shaped sigmoid colon to the anal canal. The rectum’s structure and function are tailored to the last stage of digestion: the temporary storage and controlled expulsion of fecal matter. This role is accomplished through highly distensible walls and complex muscular and nervous system controls.
The Role of Storage
The rectum’s primary mechanical function is to act as a temporary reservoir for feces before a bowel movement. This temporary holding capacity is achieved by the rectal ampulla, the widened final section of the rectum. As waste material is moved from the colon into the rectum, the rectal walls are designed to relax and stretch, a mechanism known as receptive relaxation. This distensibility allows the rectum to accumulate fecal mass without a significant immediate increase in internal pressure. Studies suggest the rectum can hold a volume up to about 300 milliliters before the pressure begins to rise substantially.
Maintaining Continence
The ability to prevent the involuntary leakage of stool, known as continence, depends heavily on the interplay of rectal sensation and surrounding muscles. The rectal walls contain specialized stretch receptors that constantly monitor the presence and volume of contents. When the rectum begins to fill, these sensory nerves transmit signals to the brain, providing the first conscious awareness of the need to defecate.
Sphincters
Fecal continence is maintained by a system of three muscle structures working together. The internal anal sphincter (IAS) is a ring of smooth muscle under involuntary control, providing 70 to 85 percent of the resting pressure in the anal canal. The external anal sphincter (EAS) is a layer of striated muscle under voluntary control, allowing a person to contract it to suppress an urge.
Puborectalis Muscle
The puborectalis muscle forms a muscular sling around the anorectal junction. This muscle maintains a sharp angle between the rectum and the anal canal, acting as a mechanical barrier to stool passage. The coordinated tone of the internal sphincter and the angulation created by the puborectalis muscle are the main forces that keep the bowel closed between elimination episodes.
The Defecation Reflex
The process of elimination is triggered when the rectum fills sufficiently to initiate the involuntary defecation reflex. As the rectum’s stretch receptors are activated by the increasing volume of stool, they send signals that cause the smooth muscle of the rectal wall to contract. Simultaneously, this reflex causes the involuntary internal anal sphincter to relax, a phenomenon called the rectoanal inhibitory reflex.
The relaxation of the internal sphincter allows a small sample of the rectal contents to move into the upper part of the anal canal. Specialized nerves in the anal lining are then able to “sample” the material, differentiating between solid, liquid, or gas. The brain receives the full sensory signal, which is the conscious urge to defecate.
If the time is appropriate, the voluntary external anal sphincter and the puborectalis muscle are consciously relaxed, straightening the anorectal angle. Contraction of the abdominal muscles increases intra-abdominal pressure, assisting the final forceful expulsion. If elimination is delayed, the rectum walls can temporarily relax, and the urge subsides until the rectum refills and re-initiates the reflex.