The sigmoid colon is the final segment of the large intestine, serving as the junction before waste material reaches the rectum. It plays an active part in the digestive process. By managing the final concentration and temporary storage of bodily waste, the sigmoid colon ensures the orderly and controlled elimination of stool. Its function involves mechanical movement and physiological absorption.
Anatomical Placement and Structure
The sigmoid colon derives its name from its physical form, which is shaped like the letter ‘S’ (or the Greek letter sigma). This segment marks the transition point between the descending colon and the rectum. It is typically positioned deep within the pelvic cavity, a location that accommodates its storage function.
This portion of the large intestine is quite mobile, suspended by a fold of tissue called the sigmoid mesocolon. This mobility allows the structure to flex and expand as it fills with waste material. The sigmoid colon measures approximately 35 to 40 centimeters (14 to 16 inches) in most adults. The S-shape and flexible attachment provide the necessary capacity to hold concentrated stool before elimination.
Primary Role in Waste Concentration
The primary role of the sigmoid colon is to act as a temporary holding chamber for formed waste material awaiting expulsion. By the time contents reach this segment, most nutrients and water have been absorbed. The sigmoid colon performs the final stage of waste concentration.
The lining of the colon absorbs the last remnants of water and electrolytes from the fecal matter. This final extraction maintains the body’s fluid balance, transforming the semi-solid material into a firm, solid mass. Fecal material can be stored here for seven hours or more, regulated by a circular muscle structure at the junction with the rectum. The inner surface is coated with a mucous membrane, which lubricates the dense waste and prevents the absorption of bacteria found within the feces.
The Sigmoid’s Role in Defecation
The sigmoid colon’s mechanical function involves the forceful propulsion of its stored contents into the rectum. The wall is composed of two layers of smooth muscle: a circular layer and longitudinal bands. These muscle layers work in a coordinated fashion to move the waste forward through infrequent but powerful contractions known as mass movements.
These mass movements occur only a few times a day and are stronger than the usual churning contractions found elsewhere in the colon. The contractions push the concentrated stool through the rectosigmoid junction and into the rectum. The arrival and resulting distension of the rectum triggers the neurological signaling known as the defecation reflex, which creates the conscious urge to evacuate.