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//VASCULAR SUPPLY AND LYMPHATIC DRAINAGE

VASCULAR SUPPLY AND LYMPHATIC DRAINAGE

Arteries

The arterial supply of the descending colon is from the inferior mesenteric artery via its left colic branch, which also anastomoses with the marginal artery of the colon (in the region of the splenic flexure), and the sigmoid arteries (in the region of the junction with the sigmoid colon) (Figs 67.29–67.31).

  

Fig. 67.29  Digital subtraction arteriogram showing the inferior mesenteric artery and its branches.
(By kind permission from Dr Adam Mitchell, Charing Cross Hospital, London.)

  

Fig. 67.30  The vascular supply of the descending colon from the inferior mesenteric artery via the ascending and descending branches of the left colic artery, coronal reformat CT.
(By kind permission from Dr Louise Moore, Chelsea and Westminster Hospital, London.)

Inferior mesenteric artery

The inferior mesenteric artery is usually smaller in calibre than the superior mesenteric artery. It arises from the anterior or left anterolateral aspect of the aorta at about the level of the third lumbar vertebra, 3 or 4 cm above the aortic bifurcation and below the third part of the duodenum. It descends in the root of the left colic mesentery initially anterior and then to the left of the aorta, crosses the origin of the left common iliac artery medial to the left ureter, and then enters, and continues in, the root of the sigmoid mesocolon as the superior rectal artery. The inferior mesenteric vein lies on its lateral side in its distal course. The principal branches of the inferior mesenteric artery are the left colic, sigmoid and superior rectal arteries (see Figs 67.11, 67.29).

Left colic artery

The left colic artery arises from the inferior mesenteric artery shortly after its origin, ascends within the left colic mesentery anterior to the left psoas major, and bifurcates into ascending and descending branches (this division may occur soon after its origin). The artery or its branches cross the left ureter and gonadal vessels. The ascending branch passes anterior to the left kidney and anastomoses with the left branch of the middle colic artery in the subperitoneal space within the transverse mesocolon. The descending branch passes laterally in the retroperitoneum and approaches the descending colon, where it forms part of the marginal artery and anastomoses with the highest sigmoid artery. The arterial arches thus formed supply the distal third of the transverse and the descending colon. Occasionally an accessory, or rarely a replaced, left colic artery may originate from the trunk of the superior mesenteric artery or the first jejunal branches. When present, it runs laterally in the upper left colic mesentery just below the level of the duodenojejunal junction to supply the upper descending colon and form part or all of the marginal artery and the arterial anastomosis in the region of the distal transverse colon (Fig. 67.32). The left colic artery may itself give rise to an accessory left middle colic artery. Occasionally the left colic artery gives rise to a branch shortly after its origin which ascends in the mesentery and anastomoses directly with a similar descending branch of the left branch of the middle colic artery (the arc of Riolan).

  

Fig. 67.32  Replaced left colic artery arising from the superior mesenteric artery.

Sigmoid (inferior left colic) arteries

There are between two and five sigmoid arteries, which are branches of the inferior mesenteric artery. They descend obliquely within the subperitoneal space in the sigmoid mesentery, anterior to the left psoas major, ureter and gonadal vessels. They supply the distal descending colon and sigmoid colon, and anastomose superiorly with the left colic artery and inferiorly with the superior rectal artery. Unlike the small intestine, arterial arcades do not form until the arteries are close to the wall of the colon, when small branches arise which supply the sigmoid colon directly. The formation of a true marginal artery is less pronounced in the region of the sigmoid colon than in the region of the descending colon. A significant space often exists in the mesentery between the highest sigmoid artery and the descending branch of the left colic artery: this is a useful guide to the arterial territories during surgical dissection.

In the region of the splenic flexure the marginal artery receives contributions from the left branch of the middle colic artery which anastomoses with an ascending branch of the left colic artery to supply the upper descending colon. The descending branch of the left colic artery anastomoses with upper branches of the highest sigmoid artery to supply the descending colon. The origin of the primary arterial supply for the splenic flexure and distal third of the transverse colon is usually via the left colic artery but may be from the left branch of the middle colic artery. The marginal artery in the region of the splenic flexure may be absent or of such small calibre as to be of little clinical relevance. It may, however, hypertrophy significantly when one of the main visceral arteries is compromised, e.g. following stenosis or occlusion of the inferior mesenteric artery, and it then provides a vessel of collateral supply.

Vascular ligation in left colonic resections

During resection of the distal descending and sigmoid colon, ligation of the inferior mesenteric artery close to its origin preserves the bifurcation of the left colic artery. The arterial supply to the proximal descending colon is maintained via the anastomosis between the ascending branch of the left colic artery and the left branch of the middle colic artery. Less radical resection, involving ligation of the left colic artery close to its bifurcation, may interfere with or obliterate this supply and render the descending colon more likely to become ischaemic. The same is true for ligation of the left colic vein. If the inferior mesenteric vein is ligated, then the bifurcation of the vein forms the route of venous drainage for the descending colon to the middle colic vein territory. Ligation of the branches separately will impair the venous drainage.

Veins

Inferior mesenteric vein

The inferior mesenteric vein drains the rectum, sigmoid, descending and distal transverse colon (Fig. 67.33; see also Fig. 67.14). It begins as the superior rectal vein, from the rectal plexus, through which it connects with middle and inferior rectal veins. The inferior mesenteric vein lies to left of the inferior mesenteric artery, ascends deep to the peritoneum and anterior to the left psoas major and may cross the testicular or ovarian vessels or ascend medial to them. It usually lies lateral to or occasionally posterior to the duodenojejunal flexure and the peritoneal fold directly lateral to the flexure is a useful guide to locating the vein surgically. It usually drains into the splenic vein, but occasionally drains into the confluence of the splenic and superior mesenteric veins or directly into the superior mesenteric vein. If a duodenal or paraduodenal fossa exists, the vein is usually in its anterior wall. The inferior mesenteric vein receives tributaries from several sigmoid veins, the middle and the left colic veins.

  

Fig. 67.33  The inferior mesenteric vein and its tributaries, digital subtraction arteriogram.
(By kind permission from Dr Adam Mitchell, Charing Cross Hospital, London.)

Left colic vein

The left colic vein is formed from several tributaries including ascending and descending branches which correspond to the equivalent arteries. These tributaries may not form a discrete vein until they drain into the inferior mesenteric vein, and occasionally there may be two distinct veins which both run into the inferior mesenteric vein. The left colic vein usually lies more superiorly in the left colon mesentery than its equivalent artery, and has a shorter course because the inferior mesenteric vein lies more laterally than the inferior mesenteric artery.

Several sigmoid veins drain the sigmoid colon and run superiorly alongside their respective arteries to drain into the inferior mesenteric vein.

Lymphatics

Lymph vessels drain from the descending colon into nodes along the left colic artery, and from the sigmoid colon into sigmoid nodes in the sigmoid mesocolon which then join superior rectal and left colic vessels. All of these nodes drain into the inferior mesenteric nodes.

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