Chapter 5: Gastrointestinal/Hepatic/Renal Physiology
Which of the following statements regarding hepatic blood supply is CORRECT?
(A) The portal vein delivers up to 75% of the total blood flow of the liver.
(B) The liver receives approximately 10% of total cardiac output.
(C) Seventy-five percent of the oxygen supply to the liver is delivered by the hepatic artery.
(D) Hepatic capillaries run alongside rows of hepatocytes in each hexagonal lobule.
(E) Zone 3 cells are most resistant to ischemia in each acinus.
The answer is A. The liver receives blood from two distinct supplies: the hepatic artery (25%–35% of total flow), which delivers blood from the general arterial circulation (via the celiac trunk) and the portal vein (65%–75% of total flow), which delivers blood from the gut and associated organs. Total flow to the liver represents approximately 25% to 30% of cardiac output, even though the liver is <3% of body weight. Under normal circumstances, oxygen delivery is shared 50:50 between the two systems, despite this difference in flow, because blood entering via the portal vein is partially deoxygenated.
Unlike most other organs, the liver lacks capillaries, and blood from the hepatic artery and portal vein mixes together in what are termed the sinusoids of the liver. These are endothelial-lined gaps that run between rows of hepatocytes. Each row converges on a so-called central vein (Figure 5-1) where blood is collected and ultimately fed to the hepatic veins. These large veins empties into the inferior vena cava, which is intimately associated with the posterior aspect of the liver. The hepatocytes, bile canaliculi and sinusoids that surround the central vein in a hexagonal pattern are known as a lobule. Portal triads occupy 4 to 5 of the 6 corners of the lobular hexagon. These triads contain branches from the hepatic artery, the portal vein, and bile ducts that carry bile in the opposite direction.
FIG. 5-1. The hepatic lobule. (Reproduced with permission from Butterworth JF IV, Mackey DC, Wasnick JD. Morgan and Mikhail's Clinical Anesthesiology. 5th ed. New York, NY: McGraw Hill; 2013.)
The functional (metabolic) unit of the liver is called the acinus. It is less clear to imagine histologically than a lobule, but incorporates an elliptical area starting at the portal triad and running along the direction of flow of the hepatic arterioles and portal venules (Figure 5-2). Cells closest to these vessels (zone 1) are well oxygenated and are least at-risk for ischemia; those in zone 3 that are furthest from the nutritive vessels are most susceptible to ischemia if oxygenation and/or perfusion pressure drops.
FIG. 5-2. Hepatic acinus with the 3 functional zones, labeled 1, 2 and 3. CV = central vein, PT = portal triad.