Portal vein

The portal vein is a large, unpaired venous trunk that conducts blood from the abdominal portion of the gastrointestinal tract (excluding the caudal part of the rectum), pancreas, and spleen to the liver. It plays a central role in the portal system, interposing a capillary bed in the liver between two venous drainage territories: the gastrointestinal viscera and the systemic circulation.

Origin and Course

The portal vein is formed by the confluence of the cranial mesenteric vein and the caudal mesenteric vein, often with the splenic vein, gastroduodenal vein, or both contributing as tributaries. This confluence typically occurs dorsal to the pancreas and near the cranial mesenteric artery.

From its origin, the portal vein courses cranially and slightly to the right, dorsal to the pancreas and adjacent to the bile duct and hepatic artery within the hepatoduodenal ligament. It enters the porta of the liver, where it divides into right and left branches, each destined for the respective liver lobes.

Tributaries

• Gastroduodenal vein: Drains the pancreas, duodenum, and pyloric stomach.

• Splenic vein: Collects blood from the spleen and may receive the left gastroepiploic and short gastric veins.

• Cranial mesenteric vein: The largest tributary, draining the jejunum, ileum, caecum, and ascending colon.

• Caudal mesenteric vein: Drains the descending colon and rectum; may empty into the cranial mesenteric or splenic vein depending on the species.

• Left gastric vein: May join the splenic vein or directly the portal vein.

• Right gastric vein: Commonly drains directly into the portal vein.

Anatomical Relations

Within the hepatoduodenal ligament, the portal vein is located dorsally relative to the bile duct (ventrally and right) and hepatic artery (left and ventral). This anatomical configuration is clinically significant during procedures such as the Pringle maneuver or during surgical exploration of the porta hepatis.

Species Variations

• In the dog, the portal vein is often formed by the convergence of the cranial mesenteric and splenic veins, with the gastroduodenal joining subsequently.

• In the ox and horse, the caudal mesenteric vein tends to join the cranial mesenteric vein, while in the pig, it may join the splenic vein.

• In ruminants, a substantial branch goes to the right lobe and right part of the quadrate lobe, while a longer left branch travels to the left lobe and the left part of the quadrate lobe.

Function

The portal vein carries nutrient-rich, oxygen-poor blood from the digestive tract to the liver. This allows hepatocytes to process absorbed nutrients, detoxify substances, and regulate blood constituents before the blood rejoins systemic circulation via the hepatic veins and caudal vena cava.

Clinical Relevance

• Portosystemic shunts, whether congenital or acquired (e.g., due to portal hypertension), involve abnormal vascular connections bypassing the liver. These are especially noted in small dog breeds.

• Portal vein thrombosis or obstruction, though rare, can lead to serious hepatic and gastrointestinal consequences.