Preglomerular resistance and glomerular perfusion in the rat and dog

Glomerular blood flow in the rat, measured with radioactive microspheres, averaged 233 +/- 59 nl/min per glomerulus, significantly less than the glomerular flow rate in the dog (568 +/- 115; P less than 0.005). The difference in glomerular blood flow rate could not be attributed to differences in mean or cortical flow rates, the fraction of acrdiac output received, cardiac output normalized to body weight, or the fractional distribution of blood flow or glomeruli from outer to inner cortex in the two species. The size of microspheres reaching the glomerulus, however, was significantly larger in the dog than in the rat (P less than 0.0005) suggesting that afferent arterioles were larger in the dog than rat. The difference in afferent resistance calculated from the size of microspheres delivered to the glomeruli was larger than the difference in glomerular blood flow. With a similar arterial pressure, a lower afferent resistance suggests a higher glomerular capillary pressure in the dog, consistent with a number of suggestions that filtration equilibrium is less likely in this species.