Hematocrit Ratio of Blood Within Mammalian Kidney and Its Significance for Renal Hemodynamics

Abstract

Evidence is presented indicating that the dynamic hematocrit of intra-renal blood is normally about one-half that in blood entering or leaving the kidney. The hematocrit ratio of intrarenal blood, relative to that in arterial blood, varies with the corpuscular concentration in arterial blood and inversely with the renal arterial blood pressure. When the blood pressure is reduced from 140 to 50 mm Hg, the vascular volume of the kidney decreases from 24% to 19% of the kidney volume and the kidney weight decreases by a like amount (i.e. 5%). Owing to the increased intrarenal hematocrit, however, the absolute quantity of red cells in kidneys removed at low pressure is usually greater than in their contralateral controls removed at high pressure. A theory is advanced to take account of the low dynamic hematocrit ratio in intrarenal blood and its variations with arterial pressure and corpuscular concentration. The theory supposes that red cells are progressively separated from plasma by a process of plasma skimming in the interlobular arteries. The deeper glomeruli are supplied primarily with plasma, leaving a highly viscous, cell-rich component of the blood to supply the terminal arterioles. After traversing the efferent arterioles, the cell-rich moiety of the blood is presumed to pass through a short circulation (preferential channels for red cells) bypassing the peritubular capillary network. The energy for the separation process is presumed to be supplied by the kinetic energy of renal arterial blood; the separation process is therefore dependent upon velocity and cell concentration. Applications of the theory to the following topics in renal physiology are discussed: a) the dynamic hematocrit of intrarenal blood; b) autoregulation of the renal circulation as a function of arterial pressure and corpuscular composition; c) afferent and efferent arteriolar resistance and the mechanism of regulation of glomerular filtration rate; d) renal extraction of PAH and Diodrast; e) oxygen supply of the kidney and its variation with blood flow.