KINETICS OF THE ELIMINATION OF SUBSTANCES INJECTED INTRAVENOUSLY (EXPERIMENTS WITH CREATININE)

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Abstract
From the proportionality between the rate of excretion and the plasma concn. of creatinine, and from the assumption that the concn. difference between plasma and tissue volume diminishes at a rate proportional to the instantaneous concn. difference, it is found that the concn. of creatinine in the plasma ([xi]) following the rapid intraven. injection of the substance is governed by the equation [xi] = ae -[alpha]t + be -[beta]t. The constants a and 6 depend on the quantity of creatinine injected, but their ratio b/a = k, as well as [alpha], [beta] and the excretion constant A, do not. Expressions have been derived to calculate the volume of fluids kinetically equivalent to plasma (P) and the tissue volume (T) in terms of these essential constants. Seven expts. on 3 dogs yielded mean values of P and T equivalent to 28 and 48% respectively of the body weight. The concn. in the tissue volume ([zeta]) is given by [zeta]=[alpha](e -[alpha]t - e -[beta]t). The maximum concn. in the tissue volume can be calculated to occur about 1 hr. after the injection; the concn. in the volume T becomes practically equal to that of P in 2 hrs. The quantity of creatinine excreted, calculated from the equation of the rate of excretion, is, on an average, 94% of the quantity injected.

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