2.1. EXTRAMURAL BLOOD AND LYMPHATIC VESSELS
The major arteries supplying the gastrointestinal tract are the celiac, superior mesenteric, and inferior mesenteric arteries. The celiac supplies the stomach and the proximal portion of the small intestine (duodenum), the superior mesenteric supplies the rest of the small intestine and proximal portion of the colon, while the inferior mesenteric supplies the distal portion of the colon. The areas supplied by these three major arterial conduits are not discrete, since there are numerous arcades of smaller arteries along the mesenteric border which anastomose with one another and provide collateral blood flow. These arcades give rise to vasa recta, whose branches encircle the musculature of the stomach, small intestine, and colon and, ultimately, penetrate the muscularis and form an arterial plexus within the submucosa [1–3].
The small veins draining the gastrointestinal tract generally parallel the arterial circuitry, including the anastomoses, and deliver the venous effluent to the portal vein via three major tributaries. The splenic vein drains the stomach, the superior mesenteric vein drains the upper small intestine, while the inferior mesenteric vein drains the distal portions of the colon. These three tributaries drain into the portal vein, which supplies the liver whose venous effluent is delivered back to the heart [2,4,5].
The lymph vessels draining the gastrointestinal tract run predominantly in association with blood vessels and enter various lymph nodes. The efferent lymphatic vessels from the lymph nodes empty into the cisterna chyli and join the systemic circulation via the thoracic duct [2,4–6].
2.2. INTRAMURAL BLOOD AND LYMPHATIC VESSELS
In general, the major arterial vessels supplying the mucosal and muscular layers of the gastrointestinal tract originate from the arterial plexus located in the submucosa. Arterioles from the submucosa branch into capillary networks in the mucosa and in the longitudinal and circular muscle layers where they run in parallel to the smooth muscle fibers. In general, the arterial supply of the mucosa and the muscularis layers of the gastrointestinal tract are arranged in parallel allowing for independent control of the blood supply to these two regions [7,8]. The lymphatic vessels draining the mucosa and the muscularis empty into the submucosal network of collecting lymphatics. In the muscularis, the lymphatic vessels run close to blood vessels with frequent anastomoses with each other .
The microcirculations of the mucosal and muscularis layers support important functional activities, such as absorption/secretion and motor activity, respectively. The mucosal layer receives approximately 80% of the total intramural blood flow; the muscularis receiving the remaining 20% [9–11]. This is presumably due to the more demanding metabolic activity of this layer. The mucosal microcirculation has a much more complex architecture than that of the muscularis, and there are some striking anatomical differences between the mucosal microcirculation of the small intestine and that of the stomach and colon.
In the stomach, submucosal arterioles branch into capillaries at the base of the glands and pass along the glands to the luminal surface of the mucosa where they form a luminal capillary network (Figure 2.1) . The capillary network surrounding the glands is drained by venules near the luminal surface of the mucosa and pass directly to the submucosal venous plexus without receiving any direct capillary tributaries within the mucosa [8,12]. When viewed from the mucosal surface by confocal endomicroscopy, the capillary networks surrounding the glands of the gastric body exhibit a honeycomb-like appearance . The gastric pits of the antrum are surrounded by a “coil-shaped” capillary network. The initial lymphatics are located below the gastric glands as a plexus. In general, no lymph vessels are found in the upper portion of the gastric mucosa [5,6,14].
In the small intestine, the submucosal arterioles enter the mucosa to form the villus microcirculation whose pattern varies among species . In general, within human villi, there is an eccentrically located single arteriole, which passes to the tip and forms a capillary fountain or tuft-like network with numerous anastomoses with the single eccentrically located venule (Figure 2.2) . The villus capillaries are situated within 2 µm of the epithelial cells . The crypt capillary network (also derived from submucosal arterioles) supplies the shaft of the villus and also drains into the venule exiting the villus.
The lymphatic system of the small intestine originates as a large centrally located vessel (lacteal) within the villi (Figure 2.3). The apical portion of the lacteal has a “cul-de-sac” endothelium, ensuring the propulsion of lymph toward the collecting lymphatics when the villus contracts. Although similar in size to the venular capillaries, they lack endothelial cell junctions, presumably to aid in the transport of chylomicra [5,17].
The colonic mucosal microvascular arrangement is similar to that of the stomach. The feeding arterioles and their capillary branches pass along the glands to the luminal surface of the mucosa where they form a capillary network surrounding the glands, presenting a honeycomb appearance when viewed from the surface [18,19]. The capillary density within the honeycomb networks is greater in the proximal colon than in the distal portion . The colonic capillaries are situated much closer to the epithelium (1 μm) than their counterparts in the small intestine . The initial lymphatics of the colon originate near the basal aspect of the gastric glands where they form plexi (Figure 2.3); the upper portion of the colonic mucosa is devoid of lymphatic vessels [16,21].