The Portal Venous System
About
The portal venous system is an essential part of the circulatory system, directing blood from the gastrointestinal tract and spleen to the liver. This pathway allows nutrients and other substances absorbed from the gut to be processed and detoxified by the liver before entering the systemic circulation/caval system. The portal venous system starts with the joining of the superior mesenteric vein (SMV) and the splenic vein, which together form the hepatic portal vein. The blood would then be filtered by the liver sinusoids and exits via the hepatic veins into the inferior vena cava. The portal system is made of veins with no valves.
Tributaries
1. Hepatic Portal Vein: This is formed by the union of the SMV and the splenic vein at the level of the L1 vertebra. It is posterior to the neck of the pancreas and first part of duodenum, anterior to the IVC. Then it ascends upwards behind the bile duct and hepatic artery to enter the liver at porta hepatis.
Inside the liver, the portal vein divides into right and left branches.
- The right branch: Before entering the right lobe of the liver will receive blood from the cystic vein
- The left branch: Enters the caudate lobe, quadrate lobe, and the left lobe of the liver.
2. Superior Mesenteric Vein (SMV): It runs parallel to the SMA and drains the small intestine, cecum, ascending colon, and part of the transverse colon. It terminates when unites with the splenic vein posterior to the neck of the pancreas to become the portal vein.
- Tributaries: The jejunal and ileal veins, right and middle colic veins, ileocolic vein, and the inferior pancreaticoduodenal vein.
3. Splenic Vein: Drains the spleen and is joined by the inferior mesenteric vein (IMV), short gastric veins, left gastroepiploic vein, and pancreatic veins. Terminates when united with the SMV to form the portal vein, posterior to the pancreas.
4. Inferior Mesenteric Vein (IMV): Drains the distal part of the colon and rectum, by receiving blood from the sigmoid veins, left colic vein, and superior rectal vein. It crosses over the left renal vein to join the portal vein at the junction where the splenic vein meets the SMV.
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5. Left and Right Gastric Veins: Drains the lesser curvature of the stomach directly into the portal vein.
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