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In medicine, the clearance is a pharmacokinetic measurement of the volume of plasma that is completely cleared off of a substance per unit time. The usual units are ml/min The total body clearance will be equal to the renal clearance + hepatic clearance + lung clearance. Although for many drugs the clearance is simply considered as the renal excretion ability, that is, the rate at which waste substances are cleared from the blood by the kidney. In these cases clearance 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 re-absorption 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.〔Kaplan Step1 Pharmacology 2010, page 14〕 The concept of clearance was described by Thomas Addis, a graduate of the University of Edinburgh Medical School. It can refer to the amount of drug removed from the whole body per unit time, or in some cases the inter-compartmental clearances can be discussed referring to redistribution between body compartments such as plasma, muscle, fat.〔 ==Definition== When referring to the function of the kidney, clearance is considered to be the ''amount of liquid filtered out of the blood that gets processed by the kidneys'' or ''the amount of blood cleaned per time'' because it has the units of a volumetric flow rate (volume / time ). However, it does not refer to a real value; "the kidney does not completely remove a substance from the total renal plasma flow."〔 Available at: (http://www.sin-italy.org/jnonline/Vol17n1/166.html ). Accessed on: Sept 2, 2007.〕 From a mass transfer perspective and physiologically, volumetric blood flow (to the dialysis machine and/or kidney) is only one of several factors that determine blood concentration and removal of a substance from the body. Other factors include the mass transfer coefficient, dialysate flow and dialysate recirculation flow for hemodialysis, and the glomerular filtration rate and the tubular reabsorption rate, for the kidney. A physiologic interpretation of clearance (at steady-state) is that clearance is ''a ratio of the mass generation and blood (or plasma) concentration''. Its definition follows from the differential equation that describes exponential decay and is used to model kidney function and hemodialysis machine function: Where: * is the mass generation rate of the substance - assumed to be a constant, i.e. not a function of time (equal to zero for foreign substances/drugs) () or () *t is dialysis time or time since injection of the substance/drug () or () *V is the volume of distribution or total body water () or () *K is the clearance () or () *C is the concentration () or () (in the USA often ()) From the above definitions it follows that is the first derivative of concentration with respect to time, i.e. the change in concentration with time. It is derived from a mass balance. Clearance of a substance is sometimes expressed as the inverse of the time constant that describes its removal rate from the body divided by its volume of distribution (or total body water). In steady-state, it is defined as the mass generation rate of a substance (which equals the mass removal rate) divided by its concentration in the blood. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Clearance (medicine)」の詳細全文を読む スポンサード リンク
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