The sigmoid colon represents the final segment of the large intestine, acting as a transition point in the digestive tract. Its role is to connect the descending colon, where waste material has been processed, to the rectum, the staging area for elimination. This muscular, flexible section is primarily responsible for preparing and holding fecal matter before a bowel movement. By managing temporary storage, the sigmoid colon ensures the final stage of excretion is regulated and controlled.
Location and Unique Structure
The sigmoid colon is situated deep within the lower left side of the abdomen and the pelvis, resting near the bladder in men and the uterus in women. It is approximately 25 to 40 centimeters long and forms a continuous path from the descending colon. This segment terminates at the third sacral vertebra, where it joins the rectum, marking the beginning of the final excretory channel.
This portion of the large intestine gets its name from the Greek letter sigma, due to its characteristic S-shaped curve. This configuration, along with its attachment by a fold of tissue called the sigmoid mesocolon, grants the segment mobility within the abdominal cavity. The flexible, looping structure enables the sigmoid colon to expand significantly, suiting it for temporary storage.
The colon’s wall contains layers of muscle, including circular and longitudinal fibers, which are responsible for its movement. The longitudinal muscles, often grouped into three bands called taeniae coli, help to propel waste forward and contribute to the formation of small pouches called haustra. This muscular arrangement allows the sigmoid colon to efficiently manage the movement and storage of increasingly solid waste material.
Primary Role in Waste Management
The physiological function of the sigmoid colon involves the final phase of waste processing and the regulation of elimination. As processed food residue reaches this section, most water and nutrient absorption has already taken place in the preceding parts of the large intestine. The sigmoid colon performs a final, minor phase of absorption, pulling trace amounts of water, electrolytes, and vitamins from the semi-solid contents.
This last-stage absorption helps to solidify the contents into firm stool, preparing them for comfortable excretion. The sigmoid colon acts as a reservoir, holding the compacted fecal matter until a convenient time for defecation. This storage can last for several hours, preventing constant or involuntary bowel movements.
The movement of waste through this segment is controlled by muscular contractions called peristalsis, which are coordinated but less frequent than in the upper digestive tract. When the sigmoid colon fills, the resulting pressure triggers nerve signals that initiate the defecation reflex. This reflex involves a reciprocal action: the sigmoid colon contracts to push material into the rectum, while the muscles at the rectosigmoid junction momentarily relax. This ensures controlled delivery of waste to the rectum, which then signals the need for evacuation.
Clinical Significance and Common Procedures
The structure and function of the sigmoid colon make it a frequent site for certain gastrointestinal conditions. Its role in storing and compacting waste means the contents here are often the densest and hardest, generating higher internal pressure on the colon wall. This increased pressure, particularly in individuals with low-fiber diets, can cause the inner lining to push through weakened spots in the muscle wall.
This process leads to the formation of small pouches known as diverticula, a condition called diverticulosis. The sigmoid colon is the most common location for these pouches to develop, affecting up to 95% of people with the condition. If these small sacs become inflamed or infected, the more serious condition of diverticulitis occurs, typically causing pain in the lower left side of the abdomen.
The sigmoid colon is also a focus during common screening and diagnostic procedures, such as a colonoscopy or a flexible sigmoidoscopy. A sigmoidoscopy is specifically used to examine the rectum and the sigmoid colon, allowing a healthcare professional to visually inspect this limited area for polyps or inflammation. For a full colonoscopy, the S-shaped curve can be one of the more challenging sections for the instrument to navigate.