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Descending colon

The descending colon is the fourth segment of the large intestine, extending from the splenic flexure (left colic flexure) to the sigmoid colon. It lies in the left lumbar region of the abdomen and runs downward along the lateral border of the left kidney to the iliac crest, where it transitions into the sigmoid colon. Unlike the transverse colon, the descending colon is retroperitoneal, covered anteriorly and laterally by peritoneum but fixed posteriorly to the abdominal wall. It functions as a site for water absorption, stool formation, and bacterial fermentation, and serves as a conduit for fecal matter toward the sigmoid colon and rectum.

Synonyms

  • Left colon

  • Colon descendens

  • Retroperitoneal colon

Function

  • Absorbs water and electrolytes from luminal contents

  • Facilitates the compaction of feces

  • Houses gut microbiota for fermentation of undigested carbohydrates

  • Propels stool into the sigmoid colon via mass peristaltic movements

Arterial Supply

  • Supplied mainly by the left colic artery, a branch of the inferior mesenteric artery (IMA)

  • Additional contribution from the sigmoid arteries (also IMA branches)

Venous Drainage

  • Drains via the left colic vein into the inferior mesenteric vein (IMV)

  • IMV joins the splenic vein, which contributes to the portal vein

Nerve Supply

  • Parasympathetic innervation: via the pelvic splanchnic nerves (S2–S4)

  • Sympathetic innervation: from the lumbar sympathetic trunk through the inferior mesenteric plexus

  • Responsible for peristaltic activity and vascular regulation

MRI Appearance

T1-weighted images:

  • Colonic wall appears low-to-intermediate signal intensity

  • Lumen may contain air (signal void, black) or fecal material with variable signal

  • Useful for assessing wall thickening, tumors, or hemorrhage

T2-weighted images:

  • Normal colonic wall: intermediate signal

  • Lumen fluid: bright hyperintense; air remains signal void

  • Wall thickening, edema, or tumors appear hyperintense relative to normal wall

STIR (Short Tau Inversion Recovery):

  • Fat suppression improves visualization of pericolic edema, inflammation, or abscesses

  • Normal wall remains low-to-intermediate, pathology appears bright hyperintense

T1 Post-Contrast (Gadolinium-enhanced):

  • Normal wall enhances homogeneously and thinly

  • Pathologies (e.g., colitis, tumors, ischemia) show heterogeneous or asymmetric enhancement

  • Especially useful for evaluating colorectal carcinoma and inflammatory bowel disease

CT Appearance

Non-contrast CT:

  • Descending colon appears as a tubular soft tissue structure along the left abdomen

  • Lumen may contain air (hypodense, black), fluid, or fecal material (mixed density)

  • Wall thickening can be assessed, though less conspicuous without contrast

Contrast-enhanced CT (CECT):

  • Colonic wall enhances, delineating wall thickening, masses, ischemia, or inflammation

  • Excellent for diagnosing diverticulitis, carcinoma, colitis, perforation, and obstruction

  • Pericolic fat stranding or abscesses are easily detected in inflammatory disease

MRI images

Descending colon anatomy MRI axial image -img-00000-00000

MRI images

Descending colon anatomy MRI coronal image -img-00000-00000

MRI image

Descending colon MRI sagittal image anatomy image -img-00000-00000

CT images

Descending colon CT axial image

CT images

Descending colon CT coronal image