The liver is the largest visceral organ in the body, weighing 1.2-1.5 kg in adults, and is relatively larger in children. The hepatic lobule, the basic histological unit, consists of a central hepatic efferent venule with cords of hepatocytes and sinusoids converging onto the efferent venule. The acinus is the functional unit and consists of a parenchymal mass between two centrilobular veins. The centre of the acinus is formed by the portal triads consisting of portal vein, hepatic artery and bile duct. The acinus is supplied by terminal branches of the hepatic artery and portal veins, which drain into sinusoids. The blood in the sinusoids drains into hepatic venules. The hepatic acinus may be divided into three functional zones: (1) periportal, (2) mediolobular and (3) centrilobular zones (Figure 6.1). Periportal hepatocytes (zone 1) receive blood with the highest
oxygen content and have the highest metabolic rate and are especially involved in protein synthesis. Centrilobular hepatocytes (zone 3) receive the least oxygen but contain high concentrations of cytochrome P-450 and therefore are important sites for drug biotransformation. These centrilobular hepatocytes are also predominantly involved in utilizing glucose, whereas periportal cells secrete glucose into the sinusoidal blood. Mediolobular hepatocytes (zone 2) receive blood with an oxygen content intermediate between zones 1 and 3 and have intermediate enzyme activities. The sinusoids form a low-pressure microcirculatory system of the acinus with sphincters at the hepatic arteriole, the hepatic venous sinusoid and arteriolar-portal shunts. Thus, the sinusoids act as a significant reservoir for blood, depending on the tone of the sphincters.