Figure 1: Traditional two-compartment model of peritoneal transport in which transfer of a drug from the peritoneal cavity to the blood occurs across the “peritoneal membrane.” The permeability-area product (PA) governs this transfer and can be calculated by measuring the rate of drug disappearance from the cavity and dividing by the overall concentration difference between the peritoneal cavity and the blood (or plasma). : the free drug concentration in the blood (or plasma); : volume of distribution of the drug in the body; : the free drug concentration in the peritoneal fluid; : volume of the peritoneal cavity. Modified from R. L. Dedrick, M. F. Flessner: pharmacokinetic problems in peritoneal drug administration: Tissue penetration and surface exposure [31].