Physiologically defined units

Reaction (12) is under control of G-6-PDH, reaction (13) is catalyzed by MHbR and provides the limiting rate, while reaction (14) is nonenzymatic and indicates the measurable oxygen uptake. The blank in this method contains no dye and the unit of MHbR system is defined as the increase of 02 uptake of one micromole per hour after the addition of an excess of toluidine blue (L7). The determination of MHbR activity in hemolyzates demands the further addition of G-6-P, G-6-PDH, and NADP to the system (W3), the enzyme activity being thus assayed independently from the physiological function of G-6-PDH system. 

In physiological terms we can also define clearance as the volume of blood cleared of the toxicant by an organ or body per unit time. Therefore, as the equations above indicate, the body clearance of a toxicant is expressed in units of volume per unit time (e.g., L/h), and can be derived if we know the volume of distribution of the toxicant... 

In the idealized framework outlined above, bacterial physiology is represented by two parameters only. The yield coefficient YBc gives the stoichiometric link between consumption of organic carbon and the limiting mineral nutrient, whereas the specific affinity constant aB defines the volume of water cleared for limiting mineral nutrient per unit bacterial biomass, per unit time. 

Most of the clinical laboratory information used to assess kidney function is derived from or related to measurement of the clearance of some substance by the kidneys. The renal clearance of a substance is defined as the volume of plasma from which the substance is completely cleared by the kidneys per unit of time. Provided a substance S is in stable concentration in the plasma is physiologically inert freely filtered at the glomerulus and neither secreted, reabsorbed, synthesized, nor metabolized by the kidney, then the amount of that substance filtered at the glomerulus is equal to the amount excreted in the urine (i.e., the amount of S entering the kidney must exactly equal the amount leaving it). The amount of S filtered at the glomerulus = GFR multiplied by plasma S concentration GFR X PS. The amount of S excreted equals the urine S concentration (US) multiplied by the urinary flow rate (V, volume excreted per unit time). 

Total body clearance (Clr) is defined as the theoretical total volume of blood, serum, or plasma completely cleared of drug per unit of time. It is usually expressed in units of mL/min, L/hr, mL/min/kg, or L/hr/kg. Like the elimination rate constant, CIt is the sum total of all the clearances contributed by each elimination route (i.e., Cfr = CIcr + CIcb + ClcM+ Clearance is a most important parameter, because it provides a better representation than does k of the body s abflity to eliminate a drug. In addition, Clj has more physiological meaning and is readily used to relate the dosing rate to steady-state concentration. 

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