Functional and anatomic heterogeneity in the liver acinus: impact on transport.

The site of solute exchange between blood and hepatocytes is the liver acinus, the structural and functional unit of hepatic parenchyma. Because the perfusion of acinar hepatocytes occurs in a sequential manner, differences in solute concentration between hepatocytes located at the inlet (zone 1) and at the outlet (zone 3) of the acinus are predictable. However, the distribution and transport of solutes by hepatocytes of each acinar zone also seem to be influenced by factors other than the sequential perfusion of blood. The concentration of receptors in hepatocytes at each zone, the presence of specific chemical groups on the incoming molecules, and the binding of the solutes to plasma protein are all factors that modify the interaction with, and the cellular concentration attained by, solutes in each acinar zone. Therefore, solute concentration in hepatocytes of each acinar zone may produce a profile of decreasing solute concentration from cells in zone 1 to those in zone 3, a reverse profile from zone 3 to zone 1, or a similar concentration in all hepatocytes.