In medicine, the clearance is a pharmacokinetic measurement of the renal excretion ability, that is, the rate at which waste substances are cleared from the blood. Although clearance may also involve other organs than the kidney, it is almost synonymous with renal clearance or renal plasma clearance. Each substance has a specific clearance that depends on its filtration characteristics. Clearance is a function of glomerular filtration, secretion from the peritubular capillaries to the nephron, and reabsorption from the nephron back to the peritubular capillaries. Clearance is variable in zero-order kinetics because a constant fraction of the drug is eliminated per unit time, but it is constant in first-order kinetics, because the amount of drug eliminated per unit time changes with the concentration of drug in the blood. The concept of clearance was described by Thomas Addis, a graduate of the University of Edinburgh Medical School.