The sigmoid colon is the final part of the colon. It appears S-shaped and is anywhere from 5.91 to 15.75 inches (15 - 40 cm) long. It begins above the pelvic inlet as a continuation of the descending colon.

The sigmoid colon starts at the level of the left iliac fossa, then makes a curve and forms an S-shape loop. When it reaches the third sacral vertebra (S3), the sigmoid colon transitions into the rectum. This transition site is known as the rectosigmoid junction. The rectosigmoid junction is also the termination site of the epiploic appendices and taeniae coli. It can be found approximately 5.91 inches or 15 cm from the anus.

Overall, the sigmoid colon has wider taeniae coli than those of the other colon parts. They become more scattered and merge together toward the distal aspect of the sigmoid colon, forming a longitudinal muscle layer that extends into the rectum and encircles its walls. The epiploic appendices are particularly more pronounced in the sigmoid colon than in other parts of the colon.

The sigmoid colon is an intraperitoneal organ and is connected to the posterior abdominal wall and posterior pelvic wall by a long mesentery called the sigmoid mesocolon. Therefore, it is quite mobile. The root of the mesentery has an inverted letter “V” shape, and it attaches the sigmoid colon to the posterior abdominal and pelvic walls.

The apex of the letter “V” is located near the site where the left common iliac artery branches into the internal iliac artery and external iliac artery. The right limb of the “V” connects the third sacral vertebra (S3) with the bifurcation of the left common iliac artery. In contrast, the left limb descends from the apex of the “V” along the medial border of the left psoas major muscle and across the external iliac vessels. And finally, it terminates at the level of the inguinal ligament.

Posterior to the sigmoid colon are the left internal iliac vessels and gonadal vessels, left ureter, piriformis muscle, sacrum, and sacral plexus. Lateral to it are the left external iliac vessels, left ovary in females or ductus deferens in males, obturator nerve, and the lateral pelvic wall. Anterior and inferior to the sigmoid colon is the urinary bladder in males and the uterus and urinary bladder in females. The loops of the ileum lie on the medial side of the sigmoid colon and are also found superior to it.

Clinical note: The sigmoid colon can twist around the mesentery, causing acute sigmoid volvulus that leads to acute obstruction and compromise of its blood supply, sometimes necessitating emergency surgery. Also, the sigmoid colon may be susceptible to out-pockets in its wall that are termed diverticula. Numerous diverticula are associated with the condition of diverticulosis. Inflammation of the diverticula is called diverticulitis.

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Sigmoid colon