The liver is a large organ (around 1500 g in the normal adult). It receives approximately 25% of the cardiac output; meaning, about 1.2 L of blood flows through the liver per minute at rest. The liver also accounts for about 20% of resting total body oxygen consumption. This organ uniquely receives a dual blood supply from the hepatic artery and portal vein (Figure 180-1).
Hepatic blood flow. (Reproduced with permission from Guyton AC. Textbook of Medical Physiology, 7th ed. W.B. Saunders: 1986.)
The hepatic artery accounts for 25% of the liver’s blood supply and delivers oxygenated blood as an arterial branch off the celiac axis. In fact, 75% of the liver’s oxygen supply comes from the hepatic artery. The biliary system and connective tissue are supplied by the hepatic artery alone whereas the rest of the liver receives the dual supply. The hepatic artery also has both alpha and beta-adrenergic receptors; therefore, flow through the artery is controlled, in part, by the splanchnic nerves of the autonomic nervous system.
In contrast, the portal vein accounts for 75% of the blood supply and 50% of the oxygen delivery. It is formed as a confluence of the splenic and superior mesenteric veins. Unlike most veins, the portal vein has no valves. It delivers blood low in oxygen but high in nutrients directly from the stomach, spleen, pancreas, and small intestine, thus giving the liver first exposure to nutrients absorbed through the gastrointestinal tract. Like the hepatic artery, portal vein blood flow is under control of the autonomic nervous system; however, it has only alpha adrenergic receptors. Normal portal venous pressures range from 5 to 10 mm Hg. Portal hypertension is defined as pressures greater than 12 mm Hg.
HEPATIC VEINS AND SINUSOIDS
Blood entering the liver parenchyma from terminal branches of the hepatic artery and portal vein mixes as it enters the hepatic sinusoids. These sinusoids are distensible vascular channels lined with endothelial cells and Kupfer cells, and bounded circumferentially by hepatocytes. These sinusoids then form the central vein of each hepatic lobule. Ultimately these central veins coalesce into the three main hepatic veins (right, left, and middle) which drain directly into the vena cava.
DETERMINANTS OF LIVER BLOOD FLOW
Portal vein blood flow is controlled primarily by the arterioles in the preportal splanchnic organs and by the resistance within the liver. Hepatic venous resistance, primarily at the level of the lobular venules (postsinusoidal), is regulated largely by the sympathetic nervous system through alpha adrenergic receptors. Hepatic arterial resistance resides primarily in the hepatic arterioles. The smooth muscle in these arterioles is affected predominantly by local ...