Ascending colon

It begins at the cecum and ends at the right colic flexure*.
It is secondarily retroperitoneal.
The right paracolic gutter lies between its lateral aspect and the abdominal wall.
Blood supply is provided by the right colic artery and the middle colic artery - both arising from the superior mesenteric artery.

*Right colic/hepatic flexure is located deep to the 9th and 10th ribs, at the inferior border of the right kidney at the L3 vertebra level, and overlapped by the part of the liver.

Transverse colon

Extends from the right to left colic flexures*.
Position is variable and is not uncommon for it to hang to the level of the umbilicus (L3).
Blood supply is provided by the middle colic artery from the superior mesenteric artery; anastomotic tributaries from the right and left colic arteries, which are from the superior mesenteric artery and inferior mesenteric artery, respectively.

*Left colic/splenic flexure lies superior (L1) to the right colic flexure and is less mobile than the right colic flexure.

Descending colon

Passes anterior to the lateral border of the left kidney.
It is positioned in the left paracolic gutter.
Blood supply is provided by the left colic artery (branch of the inferior mesenteric artery).

Sigmoid colon

S-shaped loop of variable length extending from the left iliac crest, it crosses the left iliac fossa and reaches the S2-S3 vertebral level (in the true pelvis), further continuing as the rectum.
The point at which the taeniae coli terminate indicates the rectosigmoid junction.
It is an intraperitoneal organ, and posterior to it are the left ureter and the division of the left iliac artery.
Blood supply is provided by the sigmoid arteries from the inferior mesenteric artery.

Clinical note: acute sigmoid volvulus (twisting around the mesentery) can occur, causing acute obstruction and compromise of its blood supply, sometimes necessitating emergency surgery. The sigmoid colon may be susceptible to out-pocketings in its wall termed diverticula. Numerous diverticula are associated with the condition of diverticulosis. Inflammation of diverticula is called diverticulitis.

Marginal artery

The marginal artery allows blood to flow from either direction, which is important in parts of the colon that are mobile. It can serve as a collateral channel when the aorta, superior mesenteric artery, or inferior mesenteric artery becomes occluded.

Surgical considerations:

Continuous blood supply facilitates resection and healing.
Caution needed at the left colic (splenic) flexure, where the anastomosis is between the middle colic and left colic arteries is weak.
This accounts for the high frequency of splenic flexure ischemia or infarction when the arterial supply is compromised by arteriosclerosis, embolism, or low blood flow seen in hypotension.
Surgeons must be aware of this weak anastomosis when removing sections of descending colon.

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Ascending, descending, transverse and sigmoid colon