The portal system is responsible for transporting blood from most of the gastrointestinal tract to the liver for metabolic processing before the blood returns to the heart. The portal system drains venous blood from the distal end of the esophagus, stomach, small and large intestines, proximal portion of the rectum, pancreas, and spleen. The portal system is the venous counterpart to areas supplied by the celiac trunk and the superior and inferior mesenteric arteries.
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| The portal venous system |
The liver is unique in that it receives both nutrient-rich deoxygenated blood (portal vein) and oxygenated blood (hepatic arteries). The portal vein branches as it enters the liver, where its blood percolates around hepatocytes in tiny vascular channels known as sinusoids. Hepatocytes detoxify the blood, metabolize fats, carbohydrates, and drugs, and produce bile. The sinusoids receive deoxygenated blood from the portal veins (provide blood for metabolism and detoxification) and oxygenated blood from the hepatic arteries (provide oxygen for hepatocytes). Blood exits the sinusoids into a central vein, which empties into the hepatic veins and ultimately into the inferior vena cava, which passes through the diaphragm before entering the right atrium of the heart.
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| Portal vein. |
Oral drugs travel throughout the gastrointestinal tract, where they are absorbed by the small intestine. These drugs then travel to the liver via the hepatic portal system, where they are metabolized before entering the systemic circulation.
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Because of hepatic metabolism, the concentration of oral drugs is reduced before entering the systemic circulation. This is known as the first- pass effect. Therefore, drugs that are inactivated by the liver (e.g., nitroglycerin) must be administered by a different method. For example, nitroglycerin is administered sublingually (absorption under the tongue) because, if swallowed, the liver inactivates the drug before it can enter the systemic circulation.
VEINS OF THE PORTAL SYSTEM
Veins of the portal system generally mirror the arterial branches of the celiac trunk and the superior and inferior mesenteric arteries.
The major veins of the portal system are as follows:
■ Splenic vein. Drains blood from the foregut, including the spleen, pancreas, and part of the stomach. The splenic vein courses deep to the pancreas.
■Superior mesenteric vein. Drains blood from the midgut and part of the foregut. The superior mesenteric vein is located to the right of the superior mesenteric artery as it courses over the third part of the duodenum.
♦ Gastro-omental veins. Drain blood from the greater curvature of the stomach into the superior mesenteric vein.
■ Inferior mesenteric vein. Drains blood from the hindgut, including the proximal third of the rectum. The inferior mesenteric vein usually drains into the superior mesenteric vein, inferior to its union with the portal vein.
■ Portal vein. Collects blood from the foregut, midgut, and hindgut. The portal vein is located deep to the hepatic artery and cystic duct and is formed by the union of the superior mesenteric vein and splenic vein, deep to the neck of the pancreas.• Gastric veins. Drain blood from the lesser curvature of the stomach into the portal vein.
PORTAL-CAVAL ANASTOMOSES
To better understand the portal-caval anastomoses, recall that veins in the abdomen return blood to the heart via two routes:
■ Portal system. Veins from the foregut, midgut, and hindgut drain blood to the liver before the blood enters the inferior vena cava and ultimately returns to the heart.
■ Caval system. Veins from the lower limbs, pelvis, and posterior abdominal wall transport blood directly to the inferior vena cava before the blood returns to the heart.
Portal-caval anastomoses occur at regions of the gastrointestinal tract that are drained by both the portal and systemic (- caval) systems. The principal portal-caval anastomoses are as follows:
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| The three primary portal–caval anastomoses. |
1.Distal portion of the esophagus. The left gastric vein of the hepatic portal system drains blood from the distal portion of the esophagus. However, most of the blood drained from the esophagus is through the esophageal veins, which drain into the azygos (caval) vein.
2.Anterior abdominal wall. The paraumbilical veins drain the tissue surrounding the umbilicus: Embryologically, these veins communicated with the umbilical veins. These connections may reopen during chronic portal hypertension. Normally in the adult, most of the venous drainage is from the inferior epigastric veins.
3.Rectum. The proximal portion of the rectum is drained via the superior rectal vein, which drains into the inferior mesenteric vein of the hepatic portal system. However, the remainder of the rectum is drained by the middle rectal vein (branch of the internal iliac vein) and inferior rectal vein (branch of the internal pudendal vein).
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