For a drug eliminated with first-order kinetics, clearance is a constant; that is, the ratio of rate of elimination to plasma concentration is the same regardless of plasma concentration (Figure 3–2). The magnitudes of clearance for different drugs range from a small percentage of the blood flow to a maximum of the total blood flow to the organs of elimination. Clearance depends on the drug, blood flow, and the condition of the organs of elimination in the patient. The clearance of a particular drug by an individual organ is equivalent to the extraction capability of that organ for that drug times the rate of delivery of drug to the organ. Thus, the clearance of a drug that is very effectively extracted by an organ (ie, the blood is completely cleared of the drug as it passes through the organ) is often flow-limited. For such a drug, the total clearance from the body is a function of blood flow through the eliminating organ and is limited by the blood flow to that organ. In this situation, other conditions—cardiac disease, or other drugs that change blood flow—may have more dramatic effects on clearance than disease of the organ of elimination. Note that for drugs eliminated with zero-order kinetics (see Figure 1–3, right), clearance is not constant.