Medical School · Year 1 · Anatomy Thorax Abdomen · includes a quiz and discussion video

Lecture 12: Portal Venous System

Unit 1.4: Human Gross Anatomy II - Thorax and Abdomen

Learning Objectives

By the end of this lecture, students will be able to:

1. Describe the formation and course of the portal vein
2. Identify the tributaries of the portal vein
3. Explain the concept of portosystemic anastomoses
4. Describe the clinical manifestations of portal hypertension
5. Explain the anatomy of the hepatic venous system
6. Correlate anatomical features with clinical conditions

Overview of Portal Circulation

The portal venous system represents a unique arrangement in which venous blood from the gastrointestinal tract passes through a second capillary bed in the liver before returning to the systemic circulation. This arrangement serves the critical function of delivering nutrient-rich blood from the intestines to the liver for metabolic processing, allowing first-pass metabolism of absorbed substances and detoxification before they enter the general circulation.

Blood in the portal system passes through two capillary networks. The first capillary bed is in the wall of the gastrointestinal tract, where nutrients are absorbed. The second capillary bed is formed by the hepatic sinusoids within the liver, where hepatocytes extract nutrients and toxins for processing.

Several features distinguish the portal venous system from typical venous systems. The portal veins and their tributaries contain no valves, which means blood flow direction depends entirely on pressure gradients. This lack of valves allows bidirectional flow under abnormal conditions. The portal system connects to the systemic venous system at specific anatomical sites, creating portosystemic anastomoses that become clinically significant in portal hypertension.

<image>Panel A: Blood from GI tract capillaries shown as a network in the intestinal wall absorbing nutrients and draining into mesenteric veins. Panel B: Mesenteric veins converging to form the portal vein as a large blue vessel entering the liver. Panel C: Blood flowing through hepatic sinusoids as the second capillary network shown as channels between hepatocyte plates. Panel D: Hepatic veins draining processed blood to the IVC and then to the right atrium with arrows indicating the direction of flow throughout the circuit.</image>

Formation of the Portal Vein

The portal vein is formed behind the neck of the pancreas at approximately the L2 vertebral level by the union of two major tributaries. The superior mesenteric vein carries blood from the small intestine and right colon. The splenic vein carries blood from the spleen, pancreas, and left colon (via the inferior mesenteric vein). The portal vein itself is approximately 8 centimeters in length.

Beyond these two forming tributaries, the portal vein receives several additional direct tributaries. The left gastric vein, also called the coronary vein, usually drains directly into the portal vein or the junction of the SMV and splenic vein. It drains the lesser curvature of the stomach and lower esophagus. The right gastric vein drains the pyloric region and enters the portal vein directly. The cystic vein drains the gallbladder and usually joins the portal vein or the right branch within the liver. The paraumbilical veins, small vessels in the falciform ligament connecting to the umbilicus, represent remnants of the fetal umbilical circulation and drain to the left branch of the portal vein.

<image>Panel A: The superior mesenteric vein and splenic vein in blue joining behind the neck of the pancreas at L2 to form the portal vein (8 cm in length). Panel B: The left gastric (coronary) vein draining the lesser curvature and lower esophagus and the right gastric vein draining the pyloric region entering the portal vein. Panel C: The cystic vein from the gallbladder joining the portal vein or its right branch. Panel D: The paraumbilical veins as thin blue lines in the falciform ligament connecting to the left portal branch.</image>

Superior Mesenteric Vein

The superior mesenteric vein drains the midgut derivatives, including the small intestine and the right side of the colon. It begins in the right iliac fossa by the union of veins draining the terminal ileum and cecum.

The SMV ascends in the mesentery of the small intestine, lying to the right of the superior mesenteric artery. It crosses the third part of the duodenum anteriorly and passes behind the neck of the pancreas to join the splenic vein.

The tributaries of the SMV include the jejunal and ileal veins draining the small intestine, the ileocolic vein draining the terminal ileum, cecum, and appendix, the right colic vein draining the ascending colon, the middle colic vein draining the transverse colon, the right gastroepiploic vein draining the greater curvature of the stomach, and the pancreaticoduodenal veins draining the pancreatic head and duodenum.

<image>Panel A: The SMV as a blue trunk ascending in the mesentery to the right of the SMA crossing anterior to D3 and passing behind the pancreatic neck. Panel B: Jejunal and ileal veins as multiple small branches draining the small intestine into the SMV. Panel C: The ileocolic vein from the ileocecal region, right colic vein from the ascending colon, and middle colic vein from the transverse colon. Panel D: The right gastroepiploic vein from the greater curvature of the stomach and pancreaticoduodenal veins from the pancreatic head and duodenum with arrows indicating flow toward the portal vein.</image>

Splenic Vein

The splenic vein forms at the splenic hilum from the union of tributaries draining the spleen. It courses to the right along the posterior surface of the pancreas, crossing anterior to the left kidney and aorta before joining the superior mesenteric vein behind the neck of the pancreas to form the portal vein.

The splenic vein receives several important tributaries. The short gastric veins drain the fundus of the stomach. The left gastroepiploic vein drains the left portion of the greater curvature. Pancreatic veins drain the body and tail of the pancreas throughout the splenic vein's course.

The inferior mesenteric vein deserves special attention as it drains the hindgut derivatives—the left colon and upper rectum. The IMV usually joins the splenic vein near its junction with the SMV, though anatomical variations exist in which it may join the SMV directly or at the exact confluence. The tributaries of the IMV include the left colic vein from the descending colon and splenic flexure, the sigmoid veins from the sigmoid colon, and the superior rectal vein from the upper rectum.

<image>Panel A: The splenic vein in blue forming at the splenic hilum and coursing behind the pancreas to join the SMV. Panel B: Tributaries including the short gastric veins from the gastric fundus, left gastroepiploic vein from the greater curvature, and pancreatic veins from the body and tail of the pancreas. Panel C: The inferior mesenteric vein ascending from the left colon and rectum receiving the left colic vein, sigmoid veins, and superior rectal vein. Panel D: The IMV joining the splenic vein near its termination with the aorta and left kidney shown posterior to the splenic vein.</image>

Course of the Portal Vein

From its formation behind the pancreatic neck, the portal vein ascends toward the liver in a characteristic course. It passes behind the first part of the duodenum and enters the hepatoduodenal ligament, the thickened free edge of the lesser omentum. Within this ligament, the portal vein lies posterior to the hepatic artery proper and common bile duct.

At the porta hepatis, the portal vein divides into right and left branches that enter the corresponding lobes of the liver. These branches further divide into segmental branches following the Couinaud classification, ultimately feeding into the portal venules and hepatic sinusoids.

Within the liver, the portal distribution represents a functional end distribution—there are no significant anastomoses between portal branches of different segments. This has implications for liver surgery and for understanding patterns of tumor spread.

<image>Panel A: The portal vein forming behind the pancreatic neck and ascending behind D1 of the duodenum to enter the hepatoduodenal ligament. Panel B: Cross-section of the hepatoduodenal ligament showing the portal vein posteriorly as the largest structure, hepatic artery proper on the left in red, and common bile duct on the right in green. Panel C: The portal vein dividing into right and left branches at the porta hepatis on the liver's visceral surface. Panel D: Further segmental division of the portal branches within the liver parenchyma shown semi-transparently.</image>

Portosystemic Anastomoses

Portosystemic anastomoses are connections between the portal venous system and the systemic venous system that exist at specific anatomical locations. Under normal conditions, these connections are small and clinically insignificant because portal venous pressure is low. However, when portal pressure rises, these anastomoses dilate to create alternative routes for blood to bypass the liver and return to the heart.

Five major sites of portosystemic anastomosis are recognized.

At the lower esophagus, the left gastric (coronary) vein of the portal system anastomoses with esophageal veins that drain to the azygos system of the systemic circulation. When dilated, these anastomoses produce esophageal varices, the most clinically dangerous manifestation of portal hypertension because rupture causes life-threatening hemorrhage presenting as massive hematemesis.

In the rectum, the superior rectal vein (portal, from the IMV) anastomoses with the middle and inferior rectal veins (systemic, to the internal iliac and internal pudendal veins). These can contribute to hemorrhoids, though hemorrhoids from portal hypertension are less clinically significant than esophageal varices.

In the periumbilical region, the paraumbilical veins in the falciform ligament (portal, connecting to the left portal vein) anastomose with the superficial epigastric and thoracoepigastric veins (systemic). When dilated, these produce a visible pattern of distended veins radiating from the umbilicus called caput medusae, named for the serpentine appearance resembling the head of Medusa.

At the bare area of the liver, portal venous branches anastomose with diaphragmatic veins of the systemic circulation.

In the retroperitoneum, colic, duodenal, and pancreatic veins of the portal system anastomose with lumbar and renal veins of the systemic circulation.

<image>Panel A: Lower esophageal anastomosis where the left gastric vein connects to esophageal and azygos veins with an inset showing dilated esophageal varices in cross-section, and rectal anastomosis where the superior rectal vein connects to middle and inferior rectal veins. Panel B: Periumbilical anastomosis where paraumbilical veins in the falciform ligament connect to superficial abdominal veins showing the caput medusae pattern radiating from the umbilicus. Panel C: Bare area of liver anastomosis where portal branches connect to diaphragmatic veins. Panel D: Retroperitoneal anastomosis where colic and pancreatic veins connect to lumbar and renal veins with all five sites numbered and labeled on an anterior abdominal view.</image>

Portal Hypertension

Portal hypertension is defined as an elevation of portal venous pressure above the normal range of 5-10 mmHg. Clinically significant portal hypertension, with risk of variceal bleeding, typically occurs when the portal-hepatic venous pressure gradient exceeds 10-12 mmHg.

The causes of portal hypertension are classified anatomically based on the location of obstruction to portal flow. Prehepatic causes occur before blood reaches the liver and include portal vein thrombosis and splenic vein thrombosis. Hepatic causes, representing the most common category in Western countries, occur within the liver itself; cirrhosis from any cause (alcohol, viral hepatitis, nonalcoholic fatty liver disease) is the most prevalent, but other causes include hepatic fibrosis and schistosomiasis. Posthepatic causes occur after blood leaves the liver and include Budd-Chiari syndrome (hepatic vein thrombosis), constrictive pericarditis, and right heart failure.

The consequences of portal hypertension derive from increased venous pressure and the development of collateral circulation. Splenomegaly results from venous congestion as blood backs up into the splenic vein and spleen. Ascites develops from increased hydrostatic pressure in the mesenteric capillaries combined with low oncotic pressure from decreased albumin synthesis in the diseased liver. Varices form at sites of portosystemic anastomosis as blood seeks alternative routes to the systemic circulation. Hepatic encephalopathy occurs when nitrogenous wastes including ammonia bypass the liver through collateral vessels and reach the systemic circulation, affecting brain function.

<image>Panel A: Sites of obstruction in portal hypertension: prehepatic at the portal vein with thrombosis, hepatic at the liver with cirrhotic nodules, and posthepatic at the hepatic veins with Budd-Chiari thrombosis. Panel B: Consequences including splenomegaly as an enlarged dark red spleen from congestion and dilated collateral veins at anastomotic sites including esophageal varices and caput medusae. Panel C: Ascites shown as blue shading in the peritoneal cavity from increased portal pressure and decreased albumin. Panel D: Mechanism of hepatic encephalopathy with ammonia and toxins from the gut bypassing the liver through shunts and reaching the brain.</image>

Clinical Manifestations

Esophageal varices represent the most dangerous complication of portal hypertension. These dilated submucosal veins in the lower esophagus are prone to rupture, causing massive upper gastrointestinal hemorrhage. Patients present with hematemesis, which may be massive and immediately life-threatening. Endoscopic examination reveals tortuous, dilated veins that may show red signs indicating high rupture risk. Treatment options include endoscopic band ligation, sclerotherapy, transjugular intrahepatic portosystemic shunt (TIPS), and in refractory cases, surgical portocaval shunts.

Caput medusae describes the pattern of dilated periumbilical veins that radiate from the umbilicus in patients with portal hypertension. The name refers to the serpentine appearance resembling the snakes on Medusa's head. The blood flow in these veins is away from the umbilicus, distinguishing them from the flow pattern seen in inferior vena cava obstruction.

Hemorrhoids can occur in portal hypertension from dilation of the rectal venous plexus, though clinically significant hemorrhoidal bleeding from portal hypertension is uncommon. Most hemorrhoids in patients with portal hypertension result from local factors rather than portal pressure.

Ascites is the accumulation of fluid in the peritoneal cavity. In portal hypertension, ascites is typically transudative, with low protein content. Physical examination reveals shifting dullness to percussion and a fluid wave. Paracentesis provides diagnostic information and therapeutic relief.

Hepatic encephalopathy is a neuropsychiatric syndrome resulting from the accumulation of toxins, particularly ammonia, that bypass the liver through portosystemic shunts. Clinical features range from subtle cognitive impairment and asterixis (flapping tremor) to confusion, stupor, and coma. The condition is often precipitated by GI bleeding, infection, or electrolyte disturbances.

<image>Panel A: Endoscopic view down the esophageal lumen with dilated tortuous varices as blue-purple bulging vessels and red spots indicating high bleeding risk. Panel B: Anterior view of the abdomen with caput medusae showing dilated veins radiating from the umbilicus with arrows indicating blood flow direction away from the umbilicus. Panel C: Ascites showing a distended abdomen with a percussion diagram demonstrating shifting dullness with tympany area shifting with position change. Panel D: Hepatic encephalopathy with asterixis showing a patient's extended hands with the characteristic flapping tremor in sequential wrist positions.</image>

Hepatic Veins

The hepatic veins drain the liver parenchyma directly into the inferior vena cava. Unlike the portal venous system, the hepatic veins carry blood that has been processed by the liver back to the systemic circulation.

Three main hepatic veins—right, middle, and left—converge on the IVC just below the diaphragm. They have no extrahepatic course, draining directly from the liver substance into the IVC through openings on its anterior surface.

The hepatic veins form from central veins of the hepatic lobules, which merge to form progressively larger collecting veins and ultimately the three main hepatic veins. An important anatomical point is that the hepatic veins run between the liver segments (intersegmental) rather than within them, while the portal veins run within the segments. This relationship is crucial for surgical planning when performing segmental liver resections.

Accessory hepatic veins are common, draining directly into the IVC independent of the three main veins. The caudate lobe has its own small veins that drain directly to the IVC, separate from the three major hepatic veins.

<image>Panel A: The liver shown semi-transparently containing the right, middle, and left hepatic veins in blue converging on the IVC posteriorly. Panel B: The hepatic veins running between liver segments with dashed lines indicating Couinaud segment boundaries while portal veins run within segments. Panel C: Inset cross-section at the hepatic vein-IVC junction with the three veins entering the anterior IVC wall just below the diaphragm. Panel D: Small caudate lobe veins as tiny blue branches draining separately and directly into the IVC.</image>

Budd-Chiari Syndrome

Budd-Chiari syndrome is defined as hepatic venous outflow obstruction at the level of the hepatic veins or suprahepatic IVC. This causes a posthepatic form of portal hypertension.

Causes of Budd-Chiari syndrome include hypercoagulable states (myeloproliferative neoplasms, factor V Leiden mutation, antiphospholipid syndrome), malignancy with venous invasion or compression, infections, and oral contraceptive use. In many cases, particularly in developing countries, membranous obstruction of the IVC may be the underlying cause.

Clinical features depend on the acuity and extent of obstruction. Acute presentation includes hepatomegaly, ascites, and abdominal pain, potentially progressing rapidly to liver failure. Chronic presentation may be more insidious, with progressive portal hypertension and its complications.

A characteristic anatomical feature of Budd-Chiari syndrome is caudate lobe hypertrophy. The caudate lobe drains directly into the IVC through its own small veins, bypassing the obstructed main hepatic veins. This preserved drainage allows the caudate lobe to maintain function and even hypertrophy while the rest of the liver suffers from venous congestion.

Diagnosis is made by imaging showing hepatic vein thrombosis or obstruction, with characteristic findings on CT, MRI, or ultrasound including absence of normal hepatic vein flow and caudate lobe enlargement. Treatment depends on the underlying cause and may include anticoagulation, thrombolysis, angioplasty with stenting, TIPS, or liver transplantation.

Surgical Portosystemic Shunts

Surgical and interventional procedures can decompress the portal system by creating artificial connections between the portal and systemic venous systems. These procedures reduce portal pressure and prevent variceal bleeding but may worsen hepatic encephalopathy by diverting blood away from the liver.

The portocaval shunt connects the portal vein directly to the inferior vena cava. This is highly effective at reducing portal pressure but is associated with a high rate of hepatic encephalopathy because it completely diverts portal blood from the liver.

The splenorenal shunt connects the splenic vein to the left renal vein. This procedure may be combined with splenectomy. It preserves some portal flow to the liver while providing decompression.

The mesocaval shunt connects the superior mesenteric vein to the IVC, often using an interposition H-graft. This procedure is technically easier than portocaval shunt and can be performed without dissection in the hepatic hilum.

The transjugular intrahepatic portosystemic shunt (TIPS) is currently the most commonly performed shunting procedure. This interventional radiologic technique creates a tract within the liver parenchyma, connecting the portal vein to a hepatic vein. A stent maintains the tract's patency. TIPS is less invasive than surgical shunts and has become the preferred method for portal decompression in many clinical scenarios.

<image>Panel A: Portocaval shunt with the portal vein connected directly to the IVC by an end-to-side or side-to-side anastomosis. Panel B: Splenorenal shunt with the splenic vein connected to the left renal vein with the spleen either left in place or removed. Panel C: Mesocaval shunt with the SMV connected to the IVC via an H-graft synthetic tube. Panel D: TIPS showing a stent within the liver parenchyma connecting the portal vein to a hepatic vein with the transjugular approach indicated.</image>

Clinical Correlations

Liver cirrhosis is the most common cause of portal hypertension in Western countries. Progressive fibrosis and nodular regeneration distort the hepatic architecture, increasing resistance to portal blood flow. The resulting portal hypertension leads to the development of varices, ascites, and predisposes to hepatic encephalopathy. Management focuses on treating the underlying cause when possible, preventing variceal bleeding, managing ascites, and considering liver transplantation for decompensated disease.

Portal vein thrombosis can occur as an acute or chronic condition. Acute portal vein thrombosis presents with abdominal pain and may lead to intestinal infarction if the thrombosis extends into the superior mesenteric vein. Chronic portal vein thrombosis leads to cavernous transformation, in which multiple small collateral vessels develop around the thrombosed portal vein to carry blood to the liver.

Splenic vein thrombosis, often occurring as a complication of pancreatitis, produces a distinctive pattern called "left-sided" or "sinistral" portal hypertension. Because only the splenic venous drainage is obstructed, the collateral pathway develops through the short gastric veins, leading to isolated gastric varices without esophageal varices. The varices are located in the gastric fundus rather than the esophagus. Treatment is splenectomy, which removes the source of elevated venous pressure.

Hepatorenal syndrome represents renal failure occurring in the setting of advanced liver disease and portal hypertension. The hemodynamic changes of portal hypertension lead to splanchnic vasodilation and compensatory renal vasoconstriction, resulting in decreased renal perfusion and function. Without liver transplantation, the prognosis is poor.

<image>Panel A: Cirrhotic liver with nodular brown surface and dilated engorged portal system with varices at the esophagus and periumbilical region. Panel B: Portal vein thrombosis showing acute thrombus in the portal vein lumen and chronic cavernous transformation with multiple small collateral vessels around the occluded main portal vein. Panel C: Splenic vein thrombosis with the occluded segment marked and collateral flow through dilated tortuous short gastric veins leading to isolated gastric fundal varices. Panel D: Hepatorenal syndrome showing portal hypertension causing splanchnic vasodilation which triggers compensatory renal vasoconstriction and decreased kidney perfusion.</image>

Summary

The portal vein forms behind the pancreatic neck from the union of the superior mesenteric vein and splenic vein, carrying nutrient-rich blood from the gastrointestinal tract to the liver for metabolic processing. The portal system contains no valves, allowing bidirectional flow in abnormal conditions.

Five major portosystemic anastomoses connect the portal and systemic venous systems. In portal hypertension, these dilate to form collateral pathways, with esophageal varices at the lower esophagus representing the most clinically dangerous manifestation due to the risk of life-threatening hemorrhage.

Portal hypertension causes include prehepatic (portal vein thrombosis), hepatic (cirrhosis, most common), and posthepatic (Budd-Chiari syndrome, hepatic vein thrombosis) etiologies. Clinical manifestations include splenomegaly, ascites, varices, and hepatic encephalopathy.

The hepatic veins drain the liver directly to the IVC and run between liver segments, while portal veins run within segments. In Budd-Chiari syndrome, the caudate lobe is spared because it drains directly to the IVC through separate small veins.

TIPS has become the preferred method for portal decompression in many settings, creating an intrahepatic shunt between portal and hepatic veins using a transjugular approach.

Key Terms

This content is subject to the MIT License. © 2024–2026 Hibbert School of Medicine.

Read this lecture as Markdown