6.13 : Clearance Models: Physiological Models

Drug clearance is a critical pharmacokinetic process involving the irreversible removal of drugs from the body through various organs over a specified time period. Physiological models are indispensable in determining organ-specific clearance, defined by the proportion of the drug eliminated per unit of time from the organ's blood volume.

The organ's clearance rate depends on the blood flow to the organ and the extraction ratio (E). The extraction ratio describes the organ's proficiency in drug elimination, which depends on the drug concentrations in the blood entering and exiting the organ. This ratio can vary between 0 and 1, denoting negligible to complete drug removal. For instance, an extraction ratio of 0.25 means that the organ eliminates 25% of the incoming drug.

According to the physiological model, organ clearance fluctuates with changes in blood flow and the organ's drug elimination capability. In contrast, total clearance represents a steady fraction of the blood volume from which the drug is persistently removed over a specified period. This dichotomy provides a comprehensive understanding of how the body processes drugs, contributing to effective dosage management.