ABSTRACT
The liver is composed of angular, generally hexagonal structures called lobules. In the centre of each lobule is a central vein, which is a branch of the hepatic vein and along the sides of the lobule are branches of the hepatic artery, the portal vein, bile ducts and lymph vessels. Both the portal vein and hepatic artery drain into the central vein via specialized capillaries called sinusoids. Although the lobule is the standard anatomical unit, it does not constitute a microcirculatory unit. A functional circulatory unit was described by Rappaport in 1954 and is called an acinus (Rappaport et al., 1954). The hepatic acinus is a group of cells arranged around a branch of the portal vein with its associated hepatic artery and bile duct, which drains across the acinus into the central veins of two adjacent lobules (Rappaport, 1976). The acinus is also divided into three microcirculatory zones. Zone 1 is the closest to the portal vein and thus constitutes the periportal area of the lobule. This zone receives blood rich in oxygen and nutrients, the concentrations of which fall as blood drains across the acinus to the central vein. Zones 2 and 3 are thus more prone to anoxia and toxic damage. Zone 3 roughly constitutes the centrilobular area of the lobule. This circulatory zoning appears also to be allied with some metabolic differentiation of the acinar zones.
