% of the cell's surface area. Thus, the potential for interaction between the blood and the hepatocyte is considerable.
The main structural and functional unit of the liver is the acinus. The hepatic acinus is a distinct mass of hepatocytes which are perfused unidirectionally from the terminal portal venules to the hepatic portal venules (5:2). Thus, there is sequential perfusion of liver cells from the vascular core to the acinar periphery. This unidirectional flow and sequential perfusion tend to compartmentalize the processes which occur in the acinus. Different functions are emphasized to varying degrees in different hepatocytes, depending on their spatial position. This results in the progressive modification of the blood at various distances from the portal venule (5:16).
The human liver is highly vascularized (4:95). Approximately 75% of its blood comes from the portal vein and about 25% comes from the hepatic artery. The blood from the portal vein tends to be rich in both endogenous and exogenous compounds. The liver's large size allows blood to linger within the organ. Thus, all of the blood's constituents are able to eventually make contact with the hepatocytes' plasma membranes (4:95).
In addition to plates and sinusoids, the liver also contains bile canaliculi (1:xxiv). These canaliculi are about 1 micrometer in diameter. The walls of the tubules' two irregularly shaped hemicanals are made up of adjacent hepatocytes, and represent about 13% of the hepatocytes' cell surface area (2:14). All of the canaliculi interconnect with each other. Together, they tend to form a kind of chicken wire like network within the hepatocyte plates. These tubules are the primary source of bile. The flow of bile in the canaliculi is opposite to the flow of blood in the sinusoids. The bile runs from the acinus periphery to the acinus core, whereupon it makes its way towards the duodenum (5:2).
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