hepatic clearance

Hepatic clearance measures how effectively the liver scrubs a drug out of the blood. Imagine blood flowing through the liver like water through a filter: hepatic clearance is the volume of blood that the liver completely cleans of drug per unit time.

It is governed by three factors: liver blood flow (how much blood reaches the filter), the intrinsic metabolic capacity of the hepatic enzymes (how good the filter is), and the fraction of drug that is unbound and available to be metabolized. The interplay of these factors determines the hepatic extraction ratio and, ultimately, clearance.

Understanding hepatic clearance explains why some drugs behave so differently. For high-extraction drugs, clearance tracks liver blood flow and is sensitive to changes in perfusion; for low-extraction drugs, clearance depends mainly on enzyme activity and protein binding and is highly vulnerable to enzyme induction, inhibition, and liver disease.