Dynamics of Glomerular Ultrafiltration in the Rat

This study was undertaken in an effort to examine the effects of selective variations in systemic hematocrit on the preglomerular. glomerular. and postglomerular microcirculation in the rat. By isovolemic exchange transfusions, systemic hematocrit (control 51 ml/ 100 ml) was either reduced (21 ml/100 ml, N = 7 rats) or elevated (62 ml/100 ml, N = 7). Single nephron glomerular filtration rate varied inversely and filtration fraction varied directly with the changes in hematocrit. The fall in filtration fraction with decreased hematocrit was due to a decline in the measured glomerular transcapillary hydraulic pressure difference (AP) and to a marked increase in the initial glomerular plasma flow rate. Afferent (RA) and efferent (RE) arteriolar resistances declined with the fall in hematocrit; RA fell proportionately more than did RE. The rise_in filtration fraction with the elevation in hematocrit was due to a marked increase in AP in part due to a relatively greater rise in RE than in RA . These findings provide an attractive explanation for the general tendency for filtration fraction to vary directly with hematocrit in anemic and polycythemic states in man.