M01.06.014 Colon

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Learning Objectives

Differentiate between the four parts of the colon based on position and peritoneal status.
Identify the three cardinal macroscopic features that distinguish the large bowel from the small bowel.
Master the Midgut-Hindgut transition (at the distal 1/3 of the transverse colon) and its neurovascular implications.
Understand the clinical importance of the Paracolic Gutters and the Marginal Artery of Drummond.

Anatomical Divisions & Peritoneum

The colon (150cm) forms an arch around the small intestine. Its mobility is determined by its peritoneal covering.

Section	Peritoneal Status	Key Characteristics
Ascending	Retroperitoneal	Ends at the right colic (Hepatic) Flexure.
Transverse	Intraperitoneal	Most mobile; attached via transverse mesocolon. Ends at the left colic (Splenic) Flexure.
Descending	Retroperitoneal	Passes anterior to the left kidney.
Sigmoid	Intraperitoneal	40cm “S” shape; highly mobile due to sigmoid mesocolon. Ends at S3 level.

Macroscopic Features (Large vs. Small Bowel)

To identify the colon during surgery or on imaging, look for these three features:

Teniae Coli: Three longitudinal smooth muscle ribbons. They shorten the bowel to create haustra.
Haustra: Sacculations or pouches of the colon wall.
Omental Appendices: Small, fat-filled peritoneal sacs attached to the surface.

Neurovascular Supply

The blood supply and innervation change at the transition from Midgut to Hindgut (junction of the proximal 2/3 and distal 1/3 of the transverse colon).

Midgut (SMA): Ascending & Proximal 2/3 Transverse Colon.
- Arteries: Ileocolic, Right Colic, Middle Colic.
- Nerves: Vagus Nerve (Parasympathetic).
Hindgut (IMA): Distal 1/3 Transverse, Descending, & Sigmoid Colon.
- Arteries: Left Colic, Sigmoid Arteries.
- Nerves: Pelvic Splanchnic Nerves S2-S4 (Parasympathetic).

The Marginal Artery of Drummond

A continuous arterial channel formed by anastomoses between the SMA and IMA branches. It provides collateral flow, protecting the colon during vascular occlusion.

Clinical Relevance

Paracolic Gutters: Pathways (lateral to ascending/descending colon) that allow infected fluid or bile to travel to different parts of the abdominal cavity.
Phrenicocolic Ligament: Attaches the splenic flexure to the diaphragm; it also serves to support the spleen.
Portal System: All venous blood eventually enters the Hepatic Portal Vein, ensuring toxins are processed by the liver before entering systemic circulation.