Relative flow of blood cells, platelets, and microspheres in outer and inner renal cortex

Blood platelets (Pl) may attain a relatively high marginal concentration in arterioles, perhaps also in arteries. Along a renal interlobular artery, blood passes several successive arterioles, each arteriole receiving a small flow fraction from a narrow zone adjacent to the artery wall. Thus, in theory, the Pl concentration should cumulatively decrease as the blood approaches the outer cortex, contrary to the concentration of red and white blood cells (RBC and WBC). This was tested by assessing the concentrations of these blood elements, and for comparison, the concentrations of 0.3-, 1.8- and 3.5-micron microspheres (MS), in serial blood samples from veins separately draining the outer and inner cortex in cat kidney. The outer-to-inner cortical concentration ratio was 1.28 for WBC, 1.04 for RBC, and 0.75 for Pl, confirming the working hypothesis. The RBC demargination corresponded to that of MS smaller than 1.8 micron, the demargination of WBC to that of MS larger than 3.5 micron. In contrast to Pl the even smaller 0.3-micron MS were not marginated. Thus, the margination of platelets may not be due merely to their small size.