Descending colon

Medically Reviewed by Anatomy Team

Last updated on January 24, 2025

The descending colon is a part of the large intestine that continues from the transverse colon and connects to the sigmoid colon.^[3] It primarily stores fecal matter that will eventually be evacuated. Structurally, it is a retroperitoneal organ, fixed in position, and surrounded by layers of connective tissue.

Location

The descending colon is located in the left side of the abdomen, running vertically from the left colic (splenic) flexure to the iliac fossa, where it transitions into the sigmoid colon. It lies posteriorly against the left kidney, the posterior abdominal wall, and the left iliopsoas muscle.

Anatomy

The descending colon is an integral part of the large intestine, involved in the storage and transport of fecal matter. Below is a detailed description of its anatomy:

Dimensions

The descending colon is approximately 25-30 cm in length, though its size may vary among individuals.^[1]

Structure

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

Mucosa:

The innermost layer, lined with simple columnar epithelium and goblet cells.
Goblet cells secrete mucus to facilitate the movement of fecal material.^[5]

Submucosa:

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

Muscularis Externa:

Composed of two layers of smooth muscle:

Inner circular layer: Helps with segmentation of feces.
Outer longitudinal layer: Organized into teniae coli, three longitudinal bands of smooth muscle that run along the colon.

Serosa/Adventitia:

The descending colon is mostly covered by adventitia, a connective tissue layer that anchors it to surrounding structures, since it is retroperitoneal.^[8]

Vascular Supply

Arterial Supply:

Supplied by the left colic artery and sigmoid arteries, which are branches of the inferior mesenteric artery.

Venous Drainage:

Blood drains via the inferior mesenteric vein, which empties into the portal venous system.

Lymphatic Drainage

Lymph from the descending colon drains into the inferior mesenteric lymph nodes located along the course of the inferior mesenteric artery.

Nervous Supply

Sympathetic Innervation:

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

Parasympathetic Innervation:

Provided by the pelvic splanchnic nerves (S2-S4), which stimulate peristalsis.
These nerves regulate muscle contractions and secretions.^[7]

Relationships to Surrounding Structures

Anterior: Related to coils of the small intestine and, in females, the uterus and ovaries.
Posterior: Lies against the left kidney, the posterior abdominal wall, and the left iliopsoas muscle.
Medial: Close to the inferior mesenteric artery and vein.^[6]

Peritoneal Relationships

Unlike the transverse and sigmoid colon, the descending colon is retroperitoneal, meaning it is covered by peritoneum only on its anterior and lateral surfaces.
This peritoneal configuration helps anchor it to the posterior abdominal wall, limiting its mobility.

Function

The descending colon, as part of the large intestine, has several key roles related to the final stages of digestion and waste management. Its functions are focused on the processing, storage, and transport of fecal material in preparation for elimination. Below is a detailed explanation of its functions:

Transport of Fecal Material

The descending colon moves fecal matter from the transverse colon to the sigmoid colon:

Segmental Contractions:

The inner circular muscle layer performs segmentation, breaking fecal matter into smaller portions for better processing.

Peristaltic Movements:

Coordinated contractions of the muscularis externa move fecal matter along the colon.

Mass Movements:

Strong, wave-like contractions propel fecal material toward the rectum, occurring a few times daily.

Water and Electrolyte Absorption

The descending colon absorbs residual water and electrolytes (e.g., sodium and chloride) from the chyme:

This process reduces the water content of the fecal material, making it more solid.
Electrolyte absorption helps maintain fluid and electrolyte balance in the body.^[4]

Storage of Fecal Matter

The descending colon temporarily stores fecal matter before it is transported to the sigmoid colon:

The S-shaped structure and relatively immobile position of the descending colon allow it to act as a reservoir.
The slow transit of feces gives the colon time to complete water reabsorption.

Role in Gut Microbiota Activity

The descending colon is a site of active microbial fermentation:

Beneficial gut bacteria break down undigested carbohydrates (e.g., dietary fiber) into short-chain fatty acids (SCFAs), which are absorbed and used by the body for energy.
This fermentation process also produces gases (e.g., carbon dioxide, methane), which are expelled as flatus.

Formation of Feces

The descending colon consolidates chyme into well-formed feces by removing excess water and compacting the material:

The mucus secreted by goblet cells in the mucosa helps bind the fecal material and lubricates its movement.

Immune Defense

The descending colon plays a role in immune function:

It contains gut-associated lymphoid tissue (GALT), which monitors microbial activity and helps defend against pathogens.^[2]
It maintains a balance between beneficial and harmful microorganisms in the gut.

Coordination with the Defecation Reflex

The descending colon works in tandem with the sigmoid colon and rectum during the defecation process:

Stretch receptors in the colon wall sense the accumulation of feces and trigger signals for defecation.
This reflex helps regulate bowel movements and prevent overloading of the rectum.

Separation of Gas and Fecal Material

The descending colon aids in distinguishing between gas (flatus) and fecal material:

The muscular activity of the colon ensures that gases produced during fermentation are separated and expelled appropriately without unintentional stool passage.

Clinical significance

The descending colon is prone to various gastrointestinal conditions due to its role in waste storage and transport:

Diverticulosis and Diverticulitis: The descending colon, especially in older individuals, is a common site for diverticula (small pouches in the colon wall). Inflammation or infection of these pouches (diverticulitis) can cause pain, fever, and bowel irregularities.
Colorectal Cancer: The descending colon is a frequent site for colorectal cancer. Symptoms may include changes in bowel habits, blood in the stool, and weight loss.
Obstruction: Conditions such as impacted feces, volvulus, or strictures can obstruct the descending colon, leading to severe abdominal pain, distension, and constipation.
Inflammatory Bowel Disease (IBD): Diseases like Crohn’s disease and ulcerative colitis can affect the descending colon, causing inflammation, diarrhea, and bleeding.

References

Standring, S. (2020). Gray’s Anatomy: The Anatomical Basis of Clinical Practice (42nd ed.). Elsevier. ISBN 978-0702077050.
Moore, K. L., Dalley, A. F., & Agur, A. M. R. (2017). Clinically Oriented Anatomy (8th ed.). Wolters Kluwer. ISBN 978-1496347213.
Skandalakis, J. E., Skandalakis, L. J., & Skandalakis, P. N. (2004). Surgical Anatomy and Technique: A Pocket Manual (2nd ed.). Springer. ISBN 978-0387215822.
Snell, R. S. (2012). Clinical Anatomy by Regions (9th ed.). Lippincott Williams & Wilkins. ISBN 978-1451110326.
Schuenke, M., Schulte, E., & Schumacher, U. (2010). THIEME Atlas of Anatomy: General Anatomy and Musculoskeletal System (2nd ed.). Thieme. ISBN 978-1604069228.
Borley, N. R. (2005). Last’s Anatomy: Regional and Applied (11th ed.). Churchill Livingstone. ISBN 978-0443103739.
Williams, N. S., Bulstrode, C. J. K., & O’Connell, P. R. (2018). Bailey & Love’s Short Practice of Surgery (27th ed.). CRC Press. ISBN 978-1498796507.
Netter, F. H. (2014). Atlas of Human Anatomy (6th ed.). Saunders Elsevier. ISBN 978-1455704187.