Blood flow in the liver

Every minute, 1500 ml of blood flows through the liver, with 2/3 of the blood volume entering through the portal vein and 1/3 through the hepatic artery.

Blood enters the arterial bed of the liver from the common hepatic artery (a. hepatica communis), which is a branch of the celiac arterial trunk (truncus coeliacus). The length of the common hepatic artery is 3-4 cm, the diameter is 0.5-0.8 cm.

The hepatic artery, immediately above the pylorus, not reaching 1-2 cm to the common bile duct, divides into the gastroduodenal (a. gastroduodenalis) and proper hepatic (a. hepatica propria) arteries.

The proper hepatic artery passes through the hepatoduodenal ligament, its length varies from 0.5 to 3 cm, diameter from 0.3 to 0.6 cm. Then the proper hepatic artery divides into right and left branches (this occurs directly at the porta hepatis or before entering them). The left hepatic artery supplies the left, square and caudal lobes of the liver. The right hepatic artery supplies mainly the right lobe of the liver and gives an artery to the gallbladder.

Within the liver, the branches of the hepatic artery divide dichotomously and in the final portal tracts they form terminal branches (arterioles). The diameter of an arteriole is 10-15 μm, its wall contains elastic fibers, sometimes smooth muscle fibers in the form of bundles. Thus, arterioles can form precapillary sphincters. On the periphery of the portal field, the hepatic arterioles pierce the border plate and flow directly into the sinusoids. The walls of the sinusoids are lined with endothelium. The sinusoids are located between the beams (trabeculae) of the hepatocytes. The branches of the hepatic artery give their blood to the sinusoids both on the periphery and in the center of the lobules.

The venous system of the liver is represented by veins that afferent and drain blood. The main afferent vein is the portal vein. Blood flows out of the liver through the hepatic veins, which flow into the inferior vena cava.

The portal vein (v. portae) begins at the level of the second lumbar vertebra behind the head of the pancreas. It includes two large trunks: v. lienalis and v. mesenterica superior, its largest tributaries are v. corvnaria ventriculi and v. mesenterica inferior. The length of the portal vein is 6-8 cm, the diameter is up to 1.2 cm; it has no valves, it collects blood from the unpaired organs of the abdominal cavity. At the level of the porta hepatis, the portal vein divides into the right branch, which supplies the right lobe of the liver, and the left branch, which supplies the left, caudate and square lobes of the liver.

Inside the liver, the branches of the portal vein run parallel to the branches of the hepatic artery, supplying blood to all lobes and segments of the liver, breaking down into portal venules.

The portal venule has a diameter of 20-50 µm, its walls are formed by endothelium, basement membrane and adventitial connective tissue, there are no smooth muscle bundles.

The terminal venule pierces the parenchymatous border plate and flows into the hepatic sinusoids, which thus contain mixed arterial-venous blood.

The efferent venous bed collects venous blood from the sinusoids into the central (terminal) hepatic veins, the wall of which consists of endothelium, reticulin fibers and a small amount of collagen fibers.

From the terminal hepatic venules, blood enters the sublobular and collecting veins, then into the right median and left hepatic veins, which empty into the inferior vena cava below where it passes through the opening in the tendinous portion of the diaphragm into the thoracic cavity.

At the point where they enter the inferior vena cava, the hepatic veins are closed by the circular muscle.

The portal vein is connected by numerous anastomoses with the vena cava (portocaval anastomoses). These are anastomoses with the veins of the esophagus and stomach, rectum, umbilical veins and veins of the anterior abdominal wall.

Portal hemodynamics is characterized by a gradual difference from high pressure in the mesenteric arteries to the lowest pressure in the hepatic veins.

The blood pressure in the mesenteric arteries is 120 mm Hg. Then the blood enters the capillary network of the intestine, stomach, pancreas, the pressure in this network is 10-15 mm Hg. From this network, the blood enters the venules and veins that form the portal vein, where the pressure is normally 5-10 mm Hg. From the portal vein, the blood enters the interlobular capillaries, from where it enters the hepatic venous system and passes into the inferior vena cava. The pressure in the hepatic veins fluctuates from 5 mm Hg to zero.

Normally, the pressure in the portal vein is 5-10 mm Hg or 70-140 mm H2O.

According to classical concepts, the functional-morphological unit of the liver is the hepatic lobule. The number of hepatic lobules is 500,000. The diameter of the lobule is 0.5-2 mm.

The center of the hexagonal liver lobule is the hepatic (central) vein, and the portal field is located on the periphery. The parenchyma of the lobules is formed by radially located beams (trabeculae) of hepatocytes converging to the central (hepatic) vein. Between the beams are sinusoids through which mixed arteriovenous blood flows, coming from the portal vein and hepatic artery.

At the site where the sinusoid departs from the terminal branch of the portal vein and where it enters the central vein, there are smooth muscle sphincters that regulate the flow of blood into the liver lobule.

Sinusoids are an intraorgan (intrahepatic) capillary network. Sinusoids are modified blood capillaries, unusually wide (diameter 7-21 µm), lined with endothelium.

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