Attainment of Equilibrium Between Plasma and Urine, with Reference to Measurement of Renal Clearances

Abstract

Since p-aminohippurate appears in the urine prior to phe-nolsulfonphthalein when the 2 chemicals are injd. intraven. at the same time, the time required to attain clearance equilibrium after the injn. of thiosulfate (TS),p-amimohippurate (PAH) and phenolsulfonphthalein (PSP) was detd. in the normal and pyelone-phritic kidneys from which urine was collected by ureteral catheters. Clearance values and plasma concns. are not in equilibrium until 20 min. after starting the infusion leading to constant plasma concns. This delay is attributable primarily to the time required to wash out the dead space of the renal tubules and renal pelvis, new urine being mixed with old until the latter is completely replaced. Where the plasma concn. of any substance is changing significantly, the true or equilibrium clearance cannot be calculated. PAH reaches equilibrium clearance approx. 1 min. ahead of TS and 2 to 4 min. ahead of PSP.