The sigmoid colon is part of the hindgut. It is the last part of the colon before the rectum. It acts as a site for water absorption from the faeces, as a site for flatus to be stored before being expelled, and as a site of peristalsis. In this article we will discuss the anatomy of the sigmoid colon including its blood supply, venous drainage, lymphatic drainage, and nerve supply. We will conclude with a concise summary, and review questions to test the reader’s understanding of the contents of the article.
The gastrointestinal system is divided into the foregut, midgut and hindgut. The foregut stretches from the oesophagus to the major duodenal papilla, the midgut from the major duodenal papilla to two thirds of the transverse colon, and the hindgut from this point to the pectinate line of the rectum.
The hindgut gets its blood supply from the inferior mesenteric artery, its parasympathetic nerve supply from the pelvic splanchnic nerves (S2-S4), and its sympathetic innervation from the lumbar splanchnic nerves (L1-L2). The cell bodies of the pelvic splanchnic nerves can be found in the sacral parasympathetic nucleus. Most preganglionic parasympathetic neurones synapse in the wall of the hindgut, while postganglionic neurones innervate glands and muscle to increase colonic motility and to relax the internal anal sphincter. Lumbar splanchnic nerves synapse in the abdominal aortic and inferior mesenteric plexuses to allow postganglionic sympathetic fibres to slow colonic motility and contract the internal anal sphincter. The autonomic nervous system modulates the activity of the enteric nervous system innervating the large intestine, which includes motor and sensory neurones. Meissner’s plexus can be found in the submucosa, while Auerbach’s myenteric plexus lies between the longitudinal and circular smooth muscle layers of the gut wall. Both plexuses contain interstitial cells of Cajal, which generate the pacemaker activity of the gut essential for peristalsis. Finally, visceral afferents relay the sensation of colonic distension.
The S-shaped sigmoid colon is part of the hindgut and is the last region of the large bowel before the rectum. It begins around the superior aperture of the pelvis, and is continuous with the descending colon. The sigmoid colon curves back on itself, and descends into the pelvis. It is intraperitoneal (unlike the rectum, ascending and descending colon, which are retroperitoneal) and is covered in mesentery, i.e. the sigmoid mesocolon. The mesentery is long in the center of the sigmoid colon, allowing it a significant range of mobility. The lower part of the mesentery is shorter, meaning the rectum and descending colon are relatively stable. As soon as the mesentery (sigmoid mesocolon) stops, the bowel is renamed the rectum (which is retroperitoneal). It runs deep to the lesser (or true) pelvis, together with the bladder.
It runs in front of the sacrum, external iliac vessels, the left sacral plexus, left ureter, and the left piriformis muscle (a pear shaped external hip rotator that divides the greater sciatic foramen into two openings). It also lies anterior to the posterior abdominal wall (quadratus lumborum, iliacus, iliac crest). Laterally is the vas deferens/ovary. Superior to it are loops of the ileum, and anterior to it are the bladder and uterus (in the female). It is usually around 40cm in length, lengthens with age and is a significant site of water absorption from the faeces.
The sigmoid colon usually lies within the pelvis, but can be displaced into the abdomen due to its relatively mobile nature. The sigmoid colon receives its blood supply from the two to five sigmoidal branches of the inferior mesenteric artery, which is a branch of the abdominal aorta at L3. The venous drainage of the sigmoid colon follows the arterial supply and venous blood is drained by the inferior mesenteric vein. Lymphatic drainage follows the course of the inferior mesenteric vessels.