VASCULAR SUPPLY AND LYMPHATIC DRAINAGE
The arterial supply of the large intestine is derived from both the superior and inferior mesenteric arteries. Those parts derived from the midgut (caecum, appendix, ascending colon and right two-thirds of the transverse colon) are supplied from colic branches of the superior mesenteric artery; whilst hindgut derivatives (left part of the transverse, descending and sigmoid colon, rectum and upper anal canal) are supplied predominantly from the inferior mesenteric artery, with small contributions from branches of the internal iliac artery. The larger unnamed branches of these vessels ramify between the muscular layers of the colon which they supply. They subdivide into smaller submucosal rami and enter the mucosa. The terminal branches divide into vasa brevia and vasa longa which either enter the colonic wall directly or run through the subserosa for a short distance before crossing the circular smooth muscle to give off branches to the appendices epiploicae (Fig. 76.10).
Superior mesenteric artery
The superior mesenteric artery supplies the caecum, appendix, ascending colon and right two-thirds of the transverse colon via the ileocolic, right colic and middle colic branches (Figs 76.11, 76.12). The ileocolic artery is formed as the distal continuation of the superior mesenteric artery in the root of the small bowel mesentery after the origin of the last ileal artery. Although it has many variations in its terminal distribution, it usually divides into a superior branch, which anastomoses with the right colic artery, and an inferior branch, which anastomoses with the distal superior mesenteric artery.
The right colic artery usually arises as a common trunk with the middle colic artery. Occasionally it arises separately from the right side of the superior mesenteric artery and is absent rarely. Sometimes it arises from the ileocolic when it is named the accessory right colic artery.
The middle colic artery is one of the first branches of the superior mesenteric artery and usually originates on its anterolateral aspect as a common trunk with the right colic artery. It arises just inferior to the uncinate process of the pancreas, anterior to the third part of the duodenum and ascends in the root of the transverse colon mesentery, just to the right of the midline dividing into terminal branches.
Occasionally the middle colic artery arises separately from the right colic artery. It may arise from the dorsal pancreatic artery and rarely may arise from an accessory or replaced hepatic artery arising from the superior mesenteric artery. The artery may end in left and right main branches but frequently divides into three or more main branches within the mesentery.
A large branch may be present, which runs parallel and posterior to the middle colic artery in the transverse mesocolon. This provides a direct communication between the superior and inferior mesenteric arteries and is known as the arc of Riolan.
Inferior mesenteric artery
The inferior mesenteric artery is usually smaller in calibre than the superior mesenteric artery, and 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 posterior to the horizontal part of the duodenum. It descends deep to the peritoneum, initially anterior and then to the left of the aorta. It 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. Distally the inferior mesenteric vein is lateral to it. The principal branches are the left colic, sigmoid (of which there may be several) and superior rectal arteries (Figs 76.11, 76.13).
MARGINAL ARTERY OF THE COLON (Fig. 76.12)
Figure 76.9 Appearance of the colon on multislice computerized tomographic examination. The data acquired in the axial plane can be presented in a number of ways using multiplanar and volume rendered reformatting as demonstrated below. A, Axial CT showing air within the ascending, transverse and descending colon. B, Coronal reformat from axial data set showing the caecum, ascending and descending colon. C, Volume-rendering of the colonic wall using the axial data set to produce virtual colonoscopic views to show the triangular lumen of the transverse colon. D, Volume-rendering of the air-filled colon using the axial data set to give an image similar to a double contrast barium enema. (Images provided by kind permission from GE Worldwide Medical Systems.)
E, Volume-rendering of the axial data set to produce a virtual dissection of the transverse colon.
Figure 76.10 Typical pericolic arrangement of arterial vasculature.
The marginal artery (marginal artery of Drummond) of the colon is the vessel which lies closest to and parallels the bowel wall. It is formed by the main trunks, and the arcades arising from, the ileocolic and right, middle and left colic arteries. Anastomoses form between the main terminal branches which run parallel to the colon within the mesentery and give rise to vasa recta and vasa brevia to supply the colon. In the region of the splenic flexure the marginal artery receives contributions from the left branch of the middle colic artery - a branch of the superior mesenteric artery - which ramifies and anastomoses with an ascending branch of the left colic artery to supply the upper descending colon. The descending branch of the left colic artery ramifies and 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 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.
Colonic vascular occlusion
The marginal artery of the colon may become massively dilated when there is chronic, progressive occlusion of the superior mesenteric artery, because under these conditions it is required to supply the majority of the midgut (except the proximal portion which is supplied by collateral vessels from the coeliac artery). Occlusion of the aorta or common iliac arteries may also result in dilatation of the marginal and inferior mesenteric arteries, which become an important collateral supply to the legs via dilated middle rectal vessels arising from the internal iliac artery.
Occlusion of the inferior mesenteric artery does not always result in irreversible ischaemia of the descending and sigmoid colon, because the marginal artery of the colon usually receives an adequate supply from the left branch of the middle colic artery. Moreover, the sigmoid arteries may be supplied by the superior rectal artery, which anastomoses with the middle and inferior rectal arteries. When ischaemia does occur, it is usually maximal in the proximal descending colon because this region is furthest from the collateral arterial supplies.
Vascular ligation in colonic resections
During resection of the descending and sigmoid colon, ligation of the inferior mesenteric artery close to its origin preserves the bifurcation of the left colic artery. This allows continued flow in the left colic artery to the proximal descending colon supplied by flow from the middle colic artery via the marginal 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.
VEINSThe venous drainage of the large intestine is primarily into the hepatic portal vein via the superior mesenteric and inferior mesenteric veins, although a small amount of drainage from the rectum occurs via middle rectal veins into the internal iliac vein and via inferior rectal veins into the pudendal vein. Those parts of the colon derived from the midgut (caecum, appendix, ascending colon and right two-thirds of the transverse colon) drain into colic branches of the superior mesenteric vein, whilst hindgut derivatives (left part of the transverse, descending and sigmoid colon, rectum and upper anal canal) drain into the inferior mesenteric vein.
Figure 76.11 Relations and main branches of the superior and inferior mesenteric arteries.
Figure 76.12 Digital subtraction arteriogram of the marginal artery running parallel to the colon and anastomosing with the branches of the superior mesenteric artery supplying the right side of the colon. (By kind permission from Dr J Jackson, Hammersmith Hospital, London.)
Superior mesenteric vein
The superior mesenteric vein receives middle colic, right colic and ileocolic veins. Venous blood from the wall of the caecum, appendix, ascending colon and right two-thirds of the transverse colon drains into mesenteric arcades and subsequently into segmental veins, which accompany their respective arteries. The segmental veins drain into the superior mesenteric vein, which lies to the right of the mesenteric artery. The veins tend to follow variations in arterial drainage.
Inferior mesenteric vein
The inferior mesenteric vein drains the rectum, sigmoid, descending and distal transverse colon (Figs 76.13, 76.14). It begins as the superior rectal vein, from the rectal plexus, through which it connects with middle and inferior rectal veins. The superior rectal vein leaves the pelvis and crosses the left common iliac vessels medial to the left ureter with the superior rectal artery, and continues upwards as the inferior mesenteric vein. The inferior mesenteric vein lies to left of the inferior mesenteric artery, ascending deep to the peritoneum and anterior to the left psoas major. It may cross the testicular or ovarian vessels or ascend medial to them, and then passes above, or behind, the duodenojejunal flexure. 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.
LYMPHATICSLymphatic vessels of the caecum, ascending and proximal transverse colon drain ultimately into lymph nodes related to the superior mesenteric artery, while those of the distal transverse colon, descending colon, sigmoid colon and rectum drain into nodes following the course of the inferior mesenteric artery (Fig. 76.15). In cases where the distal transverse colon or splenic flexure is predominantly supplied by vessels from the middle colic artery, the lymphatic drainage of this area may be predominantly to superior mesenteric nodes.
Figure 76.13 Digital subtraction arteriogram showing A, the inferior mesenteric artery and its branches and B, the inferior mesenteric vein and its tributaries. (By kind permission from Dr Adam Mitchell, Charing Cross Hospital, London.)
Figure 76.14 Relations and main branches of the superior and inferior mesenteric veins.
Lymph nodes related to the colon form four groups, namely epicolic, paracolic, intermediate colic and preterminal colic nodes. Epicolic nodes are minute nodules on the serosal surface of the colon, sometimes in the appendices epiploicae. Paracolic nodes lie along the medial borders of the ascending and descending colon and along the mesenteric borders of the transverse and sigmoid colon. Intermediate colic nodes lie along the named colic vessels (the ileocolic, right colic, middle colic, left colic, sigmoid and superior rectal arteries). Preterminal colic nodes lie along the main trunks of the superior and inferior mesenteric arteries and drain into para-aortic nodes at the origin of these vessels. These are commonly referred to as the highest nodes of the territory which they drain.
Lymph node clearance in colorectal cancer resections
Figure 76.15 The lymph vessels and nodes of the transverse, descending and sigmoid colon. (After Jamieson JK, Dobson JF 1908 The lymphatics of the colon. Proc R Soc Med 2: 149-174, by permission from the Royal Society of Medicine.)
Radical lymphadenectomy during resection for colorectal cancer requires removal of the highest possible lymph node draining the area of colon in which the tumour is located. In cases of cancer involving the rectum and sigmoid colon, this usually involves resection of the preterminal colic nodes of the inferior mesenteric artery and thus ligation of the inferior mesenteric artery at its root or just below the origin of the left colic artery. A detailed description of the classification of the lymph nodes with regard to the site of the primary tumour within the colon has been suggested by the Japanese Society for the Cancer of the Colon and Rectum.