Distribution of blood flow in the dog kidney: I. Saturation rates for inert diffusible tracers, ¹²⁵I‐iodoantipyrine and tritiated water, versus uptake of microspheres under control conditions

Disparate reports on intrarenal blood flow distribution prompted a direct comparison between microspheres (Ms) and inert diffusible tracers (DT). The “tissue sampling technique” for estimating local flow with DT (Kety) was adapted for the dog kidney, using ¹²⁵I‐iodoantipyrine (I‐Ap) and tritiated water (THO). Ms (15 μm) were injected 2–3 min prior to 10–15 s DT infusion made during continuous 1 s arterial blood sampling. Tracers were measured in 7 to 20 samples from each of the following zones: Outer, middle and inner cortex (C₁, C₂, C₃), outer and inner halves of outer medulla (OM₁, OM₂), and inner medulla (IM). I‐Ap and THO gave closely similar flow distributions, and average total renal blood flow (RBF) of 3.90 and 3.78 as compared to 3.94 ml/min g with Ms. Flow in C₂ (ml/min g) was similar with all tracers, and in per cent thereof average local flows were: C₁ 102, C₃ 70, OM₁ 34, OM₂ 12, and IM 2 with DT versus 117, 53, 12, 3, and 0 with Ms. Zonal flow fractions of total RBF obtained with DT were: C₁ 0.41, C₂ 0.33, and C₃+medulla 0.26 versus 0.51, 0.33 and 0.16 with Ms. Thus, a Ms surplus in C₁ relative to DT flow, representing 10% of total RBF, matched a Ms deficit in C₃+medulla. This disparity might result from: (1) Failure of Ms to enter deep afferent arterioles in proportion to blood flow, (2) diffusion of DT from deep portions of the interlobular arteries, and/or (3) postglomerular inward flow of blood and DT.