-
4. The Portal System of Veins
The portal system (Fig. 591) includes all the veins which drain the blood from the abdominal part of the digestive tube (with the exception of the lower part of the rectum) and from the spleen, pancreas, and gall-bladder. From these viscera the blood is conveyed to the liver by the portal vein. In the liver this vein ramifies like an artery and ends in capillary-like vessels termed sinusoids, from which the blood is conveyed to the inferior vena cava by the hepatic veins. From this it will be seen that the blood of the portal system passes through two sets of minute vessels, viz., (a) the capillaries of the digestive tube, spleen, pancreas, and gall-bladder; and (b) the sinusoids of the liver. In the adult the portal vein and its tributaries are destitute of valves; in the fetus and for a short time after birth valves can be demonstrated in the tributaries of the portal vein; as a rule they soon atrophy and disappear, but in some subjects they persist in a degenerate form. The portal vein (vena portæ) is about 8 cm. in length, and is formed at the level of the second lumbar vertebra by the junction of the superior mesenteric and lienal veins, the union of these veins taking place in front of the inferior vena cava and behind the neck of the pancreas. It passes upward behind the superior part of the duodenum and then ascends in the right border of the lesser omentum to the right extremity of the porta hepatis, where it divides into a right and a left branch, which accompany the corresponding branches of the hepatic artery into the substance of the liver. In the lesser omentum it is placed behind and between the common bile duct and the hepatic artery, the former lying to the right of the latter. It is surrounded by the hepatic plexus of nerves, and is accompanied by numerous lymphatic vessels and some lymph glands. The right branch of the portal vein enters the right lobe of the liver, but before doing so generally receives the cystic vein. The left branch, longer but of smaller caliber than the right, crosses the left sagittal fossa, gives branches to the caudate lobe, and then enters the left lobe of the liver. As it crosses the left sagittal fossa it is joined in front by a fibrous cord, the ligamentum teres (obliterated umbilical vein), and is united to the inferior vena cava by a second fibrous cord, the ligamentum venosum (obliterated ductus venosus). Tributaries.—The tributaries of the portal vein are: Lienal. Pyloric. Superior Mesenteric. Cystic. Coronary. Parumbilical. The Lienal Vein (v. lienalis; splenic vein) commences by five or six large branches which return the blood from the spleen. These unite to form a single vessel, which passes from left to right, grooving the upper and back part of the pancreas, below the lineal artery, and ends behind the neck of the pancreas by uniting at a right angle with the superior mesenteric to form the portal vein. The lienal vein is of large size, but is not tortuous like the artery. Tributaries.—The lineal vein receives the short gastric veins, the left gastroepiploic vein, the pancreatic veins, and the inferior mesenteric veins. The short gastric veins (vv. gastricæ breves), four or five in number, drain the fundus and left part of the greater curvature of the stomach, and pass between the two layers of the gastrolienal ligament to end in the lienal vein or in one of its large tributaries. The left gastroepiploic vein (v. gastroepiploica sinistra) receives branches from the antero-superior and postero-inferior surfaces of the stomach and from the greater omentum; it runs from right to left along the greater curvature of the stomach and ends in the commencement of the lienal vein. The pancreatic veins (vv. pancreaticæ) consist of several small vessels which drain the body and tail of the pancreas, and open into the trunk of the lienal vein. The inferior mesenteric vein (v. mesenterica inferior) returns blood from the rectum and the sigmoid, and descending parts of the colon. It begins in the rectum as the superior hemorrhoidal vein, which has its origin in the hemorrhoidal plexus, and through this plexus communicates with the middle and inferior hemorrhoidal veins. The superior hemorrhoidal vein leaves the lesser pelvis and crosses the left common iliac vessels with the superior hemorrhoidal artery, and is continued upward as the inferior mesenteric vein. This vein lies to the left of its artery, and ascends behind the peritoneum and in front of the left Psoas major; it then passes behind the body of the pancreas and opens into the lienal vein; sometimes it ends in the angle of union of the lienal and superior mesenteric veins. Tributaries.—The inferior mesenteric vein receives the sigmoid veins from the sigmoid colon and iliac colon, and the left colic vein from the descending colon and left colic flexure. The Superior Mesenteric Vein (v. mesenterica superior) returns the blood from the small intestine, from the cecum, and from the ascending and transverse portions of the colon. It begins in the right iliac fossa by the union of the veins which drain the terminal part of the ileum, the cecum, and vermiform process, and ascends between the two layers of the mesentery on the right side of the superior mesenteric artery. In its upward course it passes in front of the right ureter, the inferior vena cava, the inferior part of the duodenum, and the lower portion of the head of the pancreas. Behind the neck of the pancreas it unites with the lienal vein to form the portal vein. Tributaries.—Besides the tributaries which correspond with the branches of the superior mesenteric artery, viz., the intestinal, ileocolic, right colic, and middle colic veins, the superior mesenteric vein is joined by the right gastroepiploic and pancreaticoduodenal veins. The right gastroepiploic vein (v. gastroepiploica dextra) receives branches from the greater omentum and from the lower parts of the antero-superior and posteroinferior surfaces of the stomach; it runs from left to right along the greater curvature of the stomach between the two layers of the greater omentum. The pancreaticoduodenal veins (vv. pancreaticoduodenales) accompany their corresponding arteries; the lower of the two frequently joins the right gastroepiploic vein. The Coronary Vein (v. coronaria ventriculi; gastric vein) derives tributaries from both surfaces of the stomach; it runs from right to left along the lesser curvature of the stomach, between the two layers of the lesser omentum, to the esophageal opening of the stomach, where it receives some esophageal veins. It then turns backward and passes from left to right behind the omental bursa and ends in the portal vein. The Pyloric Vein is of small size, and runs from left to right along the pyloric portion of the lesser curvature of the stomach, between the two layers of the lesser omentum, to end in the portal vein. The Cystic Vein (v. cystica) drains the blood from the gall-bladder, and, accompanying the cystic duct, usually ends in the right branch of the portal vein. Parumbilical Veins (vv. parumbilicales).—In the course of the ligamentum teres of the liver and of the middle umbilical ligament, small veins (parumbilical) are found which establish an anastomosis between the veins of the anterior abdominal wall and the portal, hypogastric, and iliac veins. The best marked of these small veins is one which commences at the umbilicus and runs backward and upward in, or on the surface of, the ligamentum teres between the layers of the falciform ligament to end in the left portal vein. Collateral venous circulation to relieve portal obstruction in the liver may be effected by communications between (a) the gastric veins and the esophageal veins which often project as a varicose bunch into the stomach, emptying themselves into the hemiazygos vein; (b) the veins of the colon and duodenum and the left renal vein; (c) the accessory portal system of Sappey, branches of which pass in the round and falciform ligaments (particularly the latter) to unite with the epigastric and internal mammary veins, and through the diaphragmatic veins with the azygos; a single large vein, shown to be a parumbilical vein, may pass from the hilus of the liver by the round ligament to the umbilicus, producing there a bunch of prominent varicose veins known as the caput medusæ; (d) the veins of Retzius, which connect the intestinal veins with the inferior vena cava and its retroperitoneal branches; (e) the inferior mesenteric veins, and the hemorrhoidal veins that open into the hypogastrics; (f) very rarely the ductus venosus remains patent, affording a direct connection between the portal vein and the inferior vena cava.
Subscribe to the "News" RSS Feed 
TOP ۞
