Sigmoid Colon

Overview

The sigmoid colon is the S-shaped terminal segment of the large intestine that connects the descending colon to the rectum. It functions primarily as a site for the storage and propulsion of fecal matter. Its name derives from the Greek letter sigma (Σ), which it resembles in shape. The sigmoid colon plays a key role in the final stages of water absorption and stool formation before defecation.

Location

The sigmoid colon is located in the left lower quadrant of the abdomen. It:

• Extends from the end of the descending colon at the level of the pelvic brim (approximately at the iliac crest)

• Passes through the pelvic cavity, curving toward the midline

• Terminates at the rectosigmoid junction at approximately the level of the S3 vertebra

Its position and length can vary between individuals and may be affected by body habitus or colonic motility.

Structure

The sigmoid colon is:

• Approximately 40 cm in length, though this may vary

• Intraperitoneal and suspended by the sigmoid mesocolon, a fan-shaped fold of peritoneum that allows mobility

• Lined with columnar epithelium and goblet cells that secrete mucus for lubrication

• Supplied by branches of the inferior mesenteric artery, especially the sigmoid arteries

• Drained by corresponding branches of the inferior mesenteric vein

Its walls contain the typical four layers of the GI tract: mucosa, submucosa, muscularis externa (with outer longitudinal taeniae coli), and serosa.

Function

The sigmoid colon serves several digestive and mechanical functions:

• Storage of fecal matter: Acts as a temporary reservoir for stool before it is expelled

• Absorption: Continues the absorption of water, electrolytes, and some vitamins

• Transport: Uses peristaltic and mass movements to move feces toward the rectum

Physiological Role(s)

The sigmoid colon is involved in:

• Defecation reflex: When the sigmoid colon fills, stretch receptors stimulate colonic contractions and urge to defecate

• Microbial fermentation: Houses a diverse bacterial population that ferments undigested carbohydrates and produces short-chain fatty acids

• Water conservation: Final concentration of feces to form solid stool

• Mucus secretion: Protects the mucosal lining and facilitates smooth passage of fecal matter

Clinical Significance

The sigmoid colon is a common site for several gastrointestinal conditions:

• Diverticulosis: Formation of diverticula (outpouchings) is most common in the sigmoid colon due to high intraluminal pressure

• Sigmoid volvulus: Twisting of the sigmoid colon can lead to obstruction and ischemia; more common in elderly or neurologically impaired patients

• Colorectal cancer: The sigmoid colon is a frequent site for adenocarcinoma; symptoms may include altered bowel habits and rectal bleeding

• Inflammatory bowel disease: Ulcerative colitis often involves the sigmoid colon and rectum (proctosigmoiditis)

• Sigmoidoscopy: A diagnostic endoscopic procedure to visualize the rectum and sigmoid colon for polyps, tumors, or inflammation

• Diverticulitis: Inflamed or infected diverticula in the sigmoid colon can cause pain, fever, and changes in bowel movements

The sigmoid colon’s mobility, vascular supply, and role in stool regulation make it a focus in both diagnostic evaluation and surgical interventions in lower gastrointestinal disorders.