Sigmoid colon

The sigmoid colon is the S-shaped segment of the large intestine that connects the descending colon to the rectum. It plays a key role in storing fecal matter before it is expelled from the body.^[5] The sigmoid colon is named for its curved, sigmoid shape and is a mobile part of the colon due to its mesentery, allowing it to move within the abdominal cavity.^[7]

Location

The sigmoid colon is located in the lower left quadrant of the abdomen.^[3] It begins at the end of the descending colon, near the left iliac fossa, and extends medially to the midline, terminating at the rectum in the pelvic cavity.

Anatomy

The sigmoid colon is a distinct, S-shaped segment of the large intestine that connects the descending colon to the rectum. Its unique anatomical features facilitate its roles within the digestive system. Below is a detailed description of its anatomy:

Shape and Orientation

• The sigmoid colon derives its name from the Greek letter sigma (∑) due to its S-shaped curve.
• It typically has two main bends:
  • A proximal curve leading medially and anteriorly.^[1]
  • A distal curve leading back toward the midline and posteriorly into the rectum.
• The shape and length of the sigmoid colon vary among individuals, contributing to its mobility.

Length

The sigmoid colon is approximately 40 cm long, though its length can vary.

Structure

The wall of the sigmoid colon consists of the typical four layers of the gastrointestinal tract:

Mucosa:

Innermost layer, lined with simple columnar epithelium containing goblet cells for mucus secretion.

Submucosa:

Contains connective tissue, blood vessels, lymphatics, and nerves.

Muscularis Externa:

• Composed of two layers of smooth muscle:
• Inner circular layer: Responsible for segmental contractions.^[8]
• Outer longitudinal layer: Arranged in three longitudinal bands called teniae coli, which converge at the rectum.
• The muscularis layer facilitates peristalsis.

Serosa/Adventitia:

The sigmoid colon is covered by serosa, part of the peritoneum, which provides mobility.

Mesentery

• The sigmoid colon is attached to the posterior abdominal wall by the sigmoid mesocolon, a fold of peritoneum.
• This mesentery contains blood vessels, lymphatics, and nerves.
• The mobility of the sigmoid colon is attributed to this mesocolon.

Vascular Supply

Arterial Supply:

Supplied by the sigmoid arteries, branches of the inferior mesenteric artery (IMA).

Venous Drainage:

Drains into the inferior mesenteric vein (IMV), which empties into the portal venous system.

Lymphatic Drainage

Lymphatic drainage follows the blood supply, draining into the inferior mesenteric lymph nodes.^[6]

Nervous Supply

Sympathetic Innervation:

Provided by the lumbar splanchnic nerves through the inferior mesenteric plexus.

Parasympathetic Innervation:

• Provided by the pelvic splanchnic nerves (S2-S4) via the inferior hypogastric plexus.
• These nerves coordinate peristalsis and regulate its activity.

Relationship to Adjacent Structures

• Anterior: May be in contact with the bladder (in males) or the uterus and vagina (in females) as it enters the pelvic cavity.
• Posterior: Lies against the left iliopsoas muscle, left ureter, and sacrum.
• Superior: Related to the descending colon.
• Inferior: Connected to the rectum.

Function

The sigmoid colon is a vital part of the large intestine that plays a key role in the final stages of digestion and waste management. Its functions primarily involve the storage, transport, and regulation of fecal matter in preparation for elimination. Below is a detailed breakdown of its functions:

Storage of Fecal Matter

The sigmoid colon serves as a temporary reservoir for fecal material:

• After water and electrolytes are absorbed in the ascending, transverse, and descending colon, the residual waste reaches the sigmoid colon.
• The sigmoid colon holds the fecal matter until it is ready to be expelled during defecation.
• Its S-shaped structure and muscular walls are well-suited for this storage function.

Regulation of Fecal Movement

The sigmoid colon regulates the passage of stool into the rectum:

• Peristaltic contractions (rhythmic muscular movements) in the muscularis externa facilitate the slow transport of fecal matter.
• Mass movements, which are strong, coordinated peristaltic waves, occur less frequently and push fecal matter from the sigmoid colon into the rectum.

Water and Electrolyte Reabsorption

• Although most water absorption occurs earlier in the colon, the sigmoid colon plays a minor role in reabsorbing any remaining water and electrolytes.^[4]
• This ensures the stool reaches the appropriate consistency for elimination.

Coordination with the Defecation Reflex

• The sigmoid colon works in conjunction with the rectum and anus during defecation:
  • As the sigmoid colon fills with fecal material, stretch receptors in its walls trigger the defecation reflex.
  • This reflex stimulates the rectum to contract and the internal anal sphincter to relax, signaling the need to defecate.
  • Voluntary control of the external anal sphincter allows the process to be regulated consciously.

Maintenance of Bowel Regularity

• The sigmoid colon helps regulate the timing and frequency of bowel movements:
  • Its ability to store fecal matter ensures that waste is not constantly moving into the rectum, avoiding frequent defecation.
  • It plays a role in maintaining a balance between stool consistency and bowel movement frequency.

Gas Management

The sigmoid colon facilitates the expulsion of gas (flatus) by segregating gas from solid feces through its muscular activity and coordinated movements.

Immune Role

Like the rest of the colon, the sigmoid colon contains a dense population of gut-associated lymphoid tissue (GALT).

This helps monitor and respond to microbial activity, maintaining gut health and preventing infection.

Coordination with Microbiota

The sigmoid colon provides an environment for beneficial gut bacteria that:

• Ferment undigested carbohydrates, producing short-chain fatty acids (SCFAs) that are absorbed and utilized by the body.
• Contribute to stool formation and protect against harmful pathogens.

Clinical Significance

The sigmoid colon is a common site for various gastrointestinal conditions due to its role as the final segment of the large intestine before the rectum:

• Diverticulosis and Diverticulitis:
  • The sigmoid colon is the most frequent site for diverticula (small pouches in the colon wall) due to increased intraluminal pressure.^[2]
  • Inflammation or infection of these pouches causes diverticulitis, leading to abdominal pain, fever, and altered bowel habits.
• Colorectal Cancer: The sigmoid colon is a common location for colorectal cancer, which often presents with changes in bowel habits, blood in stool, or bowel obstruction.
• Volvulus: The sigmoid colon’s mobility makes it prone to twisting (sigmoid volvulus), causing intestinal obstruction, severe pain, and bloating.
• Ischemia: Reduced blood flow (ischemia) to the sigmoid colon can lead to colonic damage, often due to vascular compromise in this segment.
• Inflammatory Bowel Disease (IBD): Conditions like ulcerative colitis frequently affect the sigmoid colon, causing inflammation, bleeding, and urgency.