The large intestine, or colon, is the final segment of the digestive tract, primarily responsible for absorbing water and electrolytes before waste is eliminated. The sigmoid colon is the terminal portion, acting as a bridge between the descending colon and the rectum. Its central role is waste management, ensuring that formed fecal matter is temporarily stored and coordinated for eventual expulsion from the body.
Anatomical Placement and Structure
The sigmoid colon derives its name from its characteristic S-shape, resembling the Greek letter sigma. This segment typically measures about 35 to 40 centimeters in length and is located in the left lower quadrant of the abdomen, extending into the pelvis. It begins where the descending colon ends and connects directly to the rectum at the level of the third sacral vertebra.
A distinct feature is its attachment to the posterior abdominal wall by a fold of peritoneum called the sigmoid mesocolon. This mesentery grants the sigmoid colon a high degree of mobility, unlike other parts of the colon that are fixed. This flexibility allows the sigmoid colon to distend significantly to accommodate stored feces. The wall is composed of layers of smooth muscle tissue, which are necessary for the powerful contractions required to move waste.
Primary Function: Storage and Transport
The sigmoid colon serves as a temporary reservoir for feces before defecation. By the time waste reaches this segment, the large intestine has already absorbed most of the remaining water, electrolytes, and some vitamins. The material entering the sigmoid colon is therefore a relatively solid, formed stool.
The colon employs mass movements, which are strong, slow contractions that push waste from the descending colon into the sigmoid segment. The inherent distensibility of the sigmoid colon allows it to safely hold accumulated waste without triggering an immediate elimination reflex. Feces can be stored here for seven hours or more, or until new material requires entry. This storage function is regulated by a reflex that helps prevent premature leakage of stool into the rectum.
Coordination of Waste Elimination
The transition from storage to elimination is a highly coordinated process controlled by nervous signals. As the sigmoid colon fills with feces, the distension of its walls stimulates mechanoreceptors embedded in the lining. This stimulation triggers a reflex action that governs the controlled passage of waste into the final segment of the digestive tract.
Once a certain volume of stored material is reached, the sigmoid colon initiates a powerful contraction while simultaneously causing a relaxation of the rectosigmoid junction. This coordinated action propels the fecal bolus into the rectum. The sudden presence of stool in the rectum generates the conscious urge to defecate. The final evacuation is then under voluntary control, where the muscles of the pelvic floor and the external anal sphincter allow the stored waste to be expelled.