The portal vein system

The portal vein (of the liver) (v. portae hepatis) occupies a special place among the veins that collect blood from the internal organs. It is not only the largest visceral vein (its length is 5-6 cm, diameter 11-18 mm), but also the afferent venous link of the so-called portal system of the liver. The portal vein of the liver is located in the thickness of the hepatoduodenal ligament behind the hepatic artery and the common bile duct together with nerves, lymph nodes and vessels. It is formed from the veins of the unpaired organs of the abdominal cavity: the stomach, small and large intestines, spleen, pancreas. From these organs, venous blood flows through the portal vein to the liver, and from there through the hepatic veins to the inferior vena cava. The main tributaries of the portal vein are the superior mesenteric and splenic veins, as well as the inferior mesenteric vein, which merge with each other behind the head of the pancreas. Entering the porta hepatis, the portal vein divides into a larger right branch (r. dexter) and a left branch (r. sinister). Each of the branches of the portal vein, in turn, divides first into segmental branches, and then into branches of increasingly smaller diameters, which pass into the interlobular veins. Inside the lobules, these veins give off wide capillaries - the so-called sinusoidal vessels, flowing into the central vein. The sublobular veins emerging from each lobule merge to form three or four hepatic veins. Thus, the blood flowing into the inferior vena cava through the hepatic veins passes through two capillary networks on its way. One capillary network is located in the walls of the digestive tract, where the portal vein tributaries originate. Another capillary network is formed in the liver parenchyma from the capillaries of its lobules.

Before entering the porta hepatis (in the thickness of the hepatoduodenal ligament), the portal vein receives the cystic vein (v. cystica) from the gallbladder, the right and left gastric veins (vv. gastricae dextra et sinistra) and the prepyloric vein (v. prepylorica), which supply blood from the corresponding parts of the stomach. The left gastric vein anastomoses with the esophageal veins - tributaries of the azygos vein from the superior vena cava system. In the thickness of the round ligament of the liver, the paraumbilical veins (vv. paraumbilicales) follow to the liver. They begin in the anterior abdominal wall, in the area of the navel, where they anastomose with the superior epigastric veins - tributaries of the internal thoracic veins (from the superior vena cava system) and with the superficial and inferior epigastric veins - tributaries of the femoral and external iliac veins from the inferior vena cava system.

Tributaries of the portal vein

1. The superior mesenteric vein (v. mesentenca superior) runs at the root of the mesentery of the small intestine to the right of the artery of the same name. Its tributaries are the veins of the jejunum and ileum (vv. jejunales et ileales), the pancreatic veins (w. pancreaticael, the pancreaticoduodenal veins (vv. panсreaticoduodenales), the ileocolic vein (v. ileocolica), the right gastroepiploic vein (v. gastroomenialis dextra), the right and middle colic veins (vv. colicae media et dextra), and the vein of the appendix (v. appendicuiaris). The veins listed above bring blood to the superior mesenteric vein from the walls of the jejunum and ileum and the appendix, the ascending colon and transverse colon, from the stomach, duodenum and pancreas, and the greater omentum.
2. The splenic vein (v. splenica) is located along the upper edge of the pancreas below the splenic artery. This vein passes from left to right, crossing the aorta in front. Behind the head of the pancreas, it merges with the superior mesenteric vein. The tributaries of the splenic vein are the pancreatic veins (vv. pancieaticae), short gastric veins (vv. gastricae breves) and the left gastroepiploic vein (v. gastroomentalis sinistra). The latter anastomoses along the greater curvature of the stomach with the right vein of the same name. The splenic vein collects blood from the spleen, part of the stomach, pancreas and greater omentum.
3. The inferior mesenteric vein (v. mesenterica inferior) is formed by the fusion of the superior rectal vein (v. rectalis superior), the left colic vein (v. colica sinistra), and the sigmoid veins (vv. sigmoideae). Situated next to the left colic artery, the inferior mesenteric vein goes up, passes behind the pancreas, and flows into the splenic vein (sometimes into the superior mesenteric vein). The inferior mesenteric vein collects blood from the walls of the upper rectum, sigmoid colon, and descending colon.

In men, blood flow through the portal vein is about 1000-1200 ml/min.

Portal blood oxygen content

The oxygen content in arterial and portal blood on an empty stomach differs by only 0.4-3.3 vol.% (on average 1.9 vol.%); 40 ml of oxygen enters the liver through the portal vein every minute, which is 72% of all oxygen entering the liver.

After eating, the absorption of oxygen by the intestines increases and the difference between arterial and portal blood in oxygen content increases.

Blood flow in the portal vein

The distribution of portal blood flow in the liver is variable: blood flow to the left or right lobe of the liver may predominate. In humans, blood flow from one lobar branch system to another is possible. Portal blood flow appears to be laminar rather than turbulent.

The pressure in the portal vein in humans is normally about 7 mm Hg.

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Collateral circulation

When the outflow through the portal vein is impaired, regardless of whether it is caused by intra- or extrahepatic obstruction, portal blood flows into the central veins through venous collaterals, which then expand significantly.

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Intrahepatic obstruction (cirrhosis)

Normally, all portal blood can flow through the hepatic veins; in cirrhosis of the liver, only 13% flows out. The rest of the blood passes through collaterals, which can be combined into 4 main groups.

• Group I: collaterals passing through the area of transition of the protective epithelium into the absorbent one
  • A. In the cardiac part of the stomach there are anastomoses between the left, posterior and short veins of the stomach, which belong to the portal vein system, and the intercostal, phrenic-esophageal and hemiazygos veins, which belong to the inferior vena cava system. Redistribution of the outflowing blood into these veins leads to varicose veins of the submucosal layer of the lower esophagus and the fundus of the stomach.
  • B. In the anal area, there are anastomoses between the superior hemorrhoidal vein, which belongs to the portal vein system, and the middle and inferior hemorrhoidal veins, which belong to the inferior vena cava system. Redistribution of venous blood into these veins leads to varicose veins of the rectum.
• Group II: veins passing in the falciform ligament and associated with the umbilical veins, which are a rudiment of the fetal umbilical circulatory system.
• Group III: collaterals passing in the ligaments or folds of the peritoneum formed when it passes from the abdominal organs to the abdominal wall or retroperitoneal tissues. These collaterals pass from the liver to the diaphragm, in the splenic-renal ligament and in the omentum. They also include the lumbar veins, veins that developed in scars that formed after previous operations, as well as collaterals that form around an entero- or colostomy.
• Group IV: veins that redistribute portal venous blood into the left renal vein. Blood flow through these collaterals is carried out both directly from the splenic vein to the renal vein, and through the diaphragmatic, pancreatic, gastric veins or the vein of the left adrenal gland.

As a result, blood from the gastroesophageal and other collaterals enters the superior vena cava through the azygos or hemiazygos vein. A small amount of blood enters the inferior vena cava, and blood from the right lobar branch of the portal vein may flow into it after the formation of an intrahepatic shunt. The development of collaterals to the pulmonary veins has been described.

Extrahepatic obstruction

In extrahepatic portal vein obstruction, additional collaterals are formed, through which blood bypasses the site of obstruction in order to reach the liver. They enter the portal vein at the porta hepatis distal to the site of obstruction. These collaterals include the veins of the porta hepatis; veins accompanying the portal vein and hepatic arteries; veins passing in the ligaments that support the liver; and the diaphragmatic and omental veins. Collaterals associated with the lumbar veins can reach very large sizes.

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