Diameter of afferent arterioles during autoregulation estimated from microsphere data in the dog kidney.

Afferent arteiolar diameters, relative flow distribution, and flow conductance factors are estimated by nonlinear regression analysis of the sieving effect on microspheres in different vascular structures of the dog renal cortex. The data presented are from experiments in which microspheres of 10-30 microgram were injected into the abdominal aorta during normotension and after lowering the blood pressure to the lower limit of autoregulation. Microscopic examination of the spheres trapped in the glomeruli and the renal arteries showed an increasing exclusion of microspheres greater than 15 micrometer from the afferent arterioles during normotension. This effect was most pronounced for the deeper cortical layers and can be explained mainly as geometrical exclusion of spheres from afferent arterioles. During hypotension, progressively larger microspheres entered glomeruli and afferent arterioles, presumably due to vasodilation of the vessels. There was a significant redistribution of microspheres larger than 15 micrometer from the outer to the inner cortex during hypotension without a corresponding redistribution of smaller spheres or the estimated blood flow. Approximately the same degree of dilation of afferent arterioles was observed during autoregulatory hypotension in three cortical layers.