Effect of steric restriction on the intracortical distribution of microspheres in the dog kidney.

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Abstract
The effect on the intracortical distribution of microspheres and radioactivity caused by steric hindrance of the free movement of spheres into afferent arterioles are described by two mathematical models. The results are compared with corresponding experimental data obtained in six kidneys from normotensive dogs. The first model (A) assumes that spheres are distributed as blood flow, regardless of their size, except for those having diameters greater than that of an afferent arteriole and which do not enter this vessel. The second model (B) includes the Ferry correction. The experimental data show that the percent recovery of spheres with diameters of 20-25 mum was significantly greater in the outer cortex and significantly less in the juxtamedullary cortex than recovery of the smaller spheres, and that the distribution of spheres with diameters of 10 mum to about 17 mum seems uninfluenced by the sphere size. The experimental results we have obtained fit best with model A. We found that according to both models steric restriction is a factor of major importance in relation to the intracortical distribution of spheres, and the analysis shows that the blood flow in the inner part of the renal cortex is grossly underestimated by the method of isotope labeled microspheres when diameters of 15 +/- 5 mum are used in the dog. Furthermore we found that dilation of the afferent arterioles will change the steric hindrance so that a redistribution of spheres and radioactivity may occur without any redistribution of blood flow. It is suggested that the results interpreted as redistribution of blood flow can be explained as due to altered steric hindrance alone, i.e., as a methodological artifact.