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//VASCULAR SUPPLY AND LYMPHATIC DRAINAGE

VASCULAR SUPPLY AND LYMPHATIC DRAINAGE

Arteries

Branches from the superior mesenteric artery supply the jejunum and ileum. The arteries divide as they approach the mesenteric border, giving off numerous branches which extend between the serosal and muscular layers, supplying the muscle and forming an intricate sub-mucosal plexus that supplies the glands and villi. Although there is a profuse anastomotic network of arteries within the mesentery, anastomoses between the terminal branches close to the intestinal wall are few (Fig. 66.8). The intramural and submucosal arterial networks are not of large calibre, which means that flow along lengths of small bowel is limited to a few centimetres, and division or occlusion of several consecutive vasa recta may produce segmental, focal ischaemia. Division of more proximal vessels allows flow to continue in the arcades for moderately long lengths of intestine.

  

Fig. 66.8  Specimens of the jejunum (A) and ileum (B) from a cadaver where the superior mesenteric artery was injected with a red coloured mass of gelatin before fixation. Subsequently the specimens were dehydrated and then cleared in benzene followed by methyl salicylate. The largest vessels present are the jejunal and ileal branches of the superior mesenteric artery and these are succeeded by anastomotic arterial arcades, which are relatively few in number (1–3) in the jejunum, becoming more numerous (5–6) in the ileum. From the arcades, straight arteries pass towards the gut wall; frequently, successive straight arteries are distributed to opposite sides of the gut. Note the denser vascularity of the jejunal wall.
(Specimens prepared by MCE Hutchinson; photographs by Kevin Fitzpatrick on behalf of GKT School of Medicine, London.)

Superior mesenteric artery

The superior mesenteric artery originates from the aorta 1 cm below the coeliac trunk, at the level of the intervertebral disk between the first and second lumbar vertebrae (Figs 66.9, 66.10). The angle of its origin from the aorta is acute, which makes cannulation via the transfemoral route somewhat difficult. The artery runs inferiorly and anteriorly, anterior to the uncinate process of the pancreas and the third part of the duodenum, and posterior to the splenic vein and the body of the pancreas. The left renal vein lies behind it and separates it from the aorta (Fig. 66.11). The artery crosses anterior to the inferior vena cava, right ureter and right psoas major as it descends in the root of the small bowel mesentery. Its calibre progressively decreases as successive branches are given off to loops of jejunum and ileum, and its terminal branch anastomoses with the ileocolic artery.

  

Fig. 66.9  The superior mesenteric artery and its branches. The outlines of representative ileal and jejunal loops, appendix, caecum, ascending and transverse colon are shown for reference. Only the origin of jejunal and ileal branches are shown. For details of arcades see Fig. 66.8.

  

Fig. 66.10  The superior mesenteric artery and its branches. A, Digital subtraction angiogram. C, Sagittal reformat of multislice CT angiogram.
(by courtesy of Dr Adam Mitchell, Charing Cross Hospital, London). B, Surface shaded, volume rendered CT angiogram (by courtesy of Dr Nasir Khan, Chelsea and Westminster Hospital, London) (by courtesy of GE Worldwide Medical Systems)

  

Fig. 66.11  Ultrasound image through the origin of the superior mesenteric artery, sagittal plane.

The superior mesenteric artery gives off middle colic, right colic (sometimes), ileocolic, jejunal and ileal branches. It may be the source of the common hepatic, gastroduodenal, accessory right hepatic, accessory pancreatic or splenic arteries, and may arise from a common coeliacomesenteric trunk. Its jejunal and ileal branches form arcades within the mesentery to a varying extent. The last of these arcades is said to form a ‘marginal artery’ of the small intestine, although the latter is irregular and not a single definable vessel. Straight arteries, the vasa recta, which supply the small intestine directly without further branches, are given off from this lowest arcade.

The remnant of the vitellointestinal artery (the embryonic artery which originally connected the intestinal circulation to the yolk sac) is usually obliterated; when present, it forms the artery supplying a Meckel’s diverticulum. It is occasionally represented in the mesentery by a fibrous strand from the region of the last ileal branch.

Jejunal branches

There are usually five to ten jejunal branches which arise from the left side of the upper portion of the superior mesenteric artery (Figs 66.9, 66.10). They are distributed to the jejunum as a series of short arcades which form a single (occasionally double) tier of anastomotic arcs before giving rise to multiple vasa recta (Fig. 66.8). These vessels run almost parallel in the mesentery and are distributed alternately to opposite aspects of its wall where the two series of vessels form distinct ‘leaves’ within the mesentery. Small twigs supply regional lymph nodes and other structures in the mesentery.

Ileal branches

Ileal branches are more numerous than the jejunal branches but smaller in calibre. They arise from the left and anterior aspects of the superior mesenteric artery. The length of the mesentery is greater in the ileum and the branches form three, four or sometimes five tiers of arcs within the mesentery before giving rise to multiple vasa recta that run directly towards the ileal wall. The ileal branches run parallel in the mesentery and are distributed to alternate aspects of the ileum. They are longer and smaller than similar jejunal vessels, particularly in the distal ileum, and do not form such definite parallel ‘leaves’ of vessels. The vascular supply in the last loop of the terminal ileum is limited. There are usually only two separate arcades, peri-serosal and in the mid-zone of the mesentery. They each receive a contribution from the ileal branch of the ileocolic artery and the last ileal branch of the superior mesenteric artery and are often larger in calibre than the mid-ileal vessels. Other than this connection, few if any vessels connect the ileocolic and superior mesenteric arteries, which makes surgical exposure of the ileocolic artery up to its origin relatively simple.

Veins

Superior mesenteric vein

The superior mesenteric vein drains the small intestine, caecum, ascending and transverse parts of the colon (Fig. 66.12; see Fig. 60.8). It is formed in the right lower mesentery of the small bowel by the union of tributaries from the terminal ileum, caecum and vermiform appendix. It ascends in the mesentery to the right of the superior mesenteric artery, passes anterior to the right ureter, inferior vena cava, third part of the duodenum and uncinate process of the pancreas, and finally joins the splenic vein behind the neck of the pancreas to form the portal vein.

  

Fig. 66.12  The superior mesenteric vein and its branches, CT venogram, coronal plane.
(Courtesy of Dr Nasir Khan, Chelsea and Westminster Hospital, London.)

Tributaries

The superior mesenteric vein receives jejunal, ileal, ileocolic, right colic (when present), middle colic, right gastroepiploic and pancreaticoduodenal veins in a similar distribution to the corresponding arteries.

Lymphatics

Lymph vessels, lacteals, are found in the mucosa and muscular coat of the small bowel. Lacteals from the villi arise from an intricate plexus in the mucosa and submucosa and are joined by vessels from lymph spaces at the bases of solitary lymphoid follicles. They drain to larger vessels at the mesenteric border of the gut. The lacteals of the muscular tunic form a close plexus that runs mostly between the two muscle layers; they communicate freely with mucosal vessels and open into vessels at the mesenteric border. Mesenteric lacteals pass between the layers of the mesentery and drain into a series of mesenteric lymph nodes which are arranged in tiers within the mesentery and which follow the same distribution as the regional arterial supply; they may form ‘chains’ along the major arteries. Elsewhere in the ileal and jejunal mesenteries the lacteals form an extensive network that affords a relatively wide field of lymph node drainage. This arrangement makes radical surgical resection of lymph nodes difficult if the vessels to the remaining unaffected small bowel are to be preserved. The mesenteric nodes drain into superior mesenteric nodes around the root of the superior mesenteric artery.

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