Functional heterogeneity of nephrons. II. Filtration rates, intraluminal flow velocities and fractional water reabsorption.

Abstract

Non-diuretic rats and saline diuretic psammomys received 0.05 ml14C Na ferrocyanide i.v. Their kidneys were frozen in vivo 8–10 sec later. The quantity of14C ferrocyanide precipitated as prussian blue (Hanssen) in the lumen of a microdissected proximal tubule was proportional to its glomerular filtration rate (gfr). Distance between dye front and glomerulus was proportional to mean flow velocity (v). Fractional water reabsorption at the dye front was calculated using gfr, v and luminal radius. There were large differences in gfr, v and fractional water reabsorption between superficial and deep nephrons. Published micropuncture data agrees quantitatively with our results. Glomerular volume, tubular length and luminal radius increased from superficial to juxtamedullary proximal tubules in both species. Filtration rate varied directly with glomerular volume suggesting an influence of capillary surface area on filtration. Tubular length and luminal radius correlated with gfr. Proximal water reabsorption rate correlated with luminal radius in psammomys but not in rat tubules. Water reabsorption by the entire proximal tubule decreased with length in rat tubules but increased in psammomys. The discrepancy may be due to differences in blood flow distribution which alter gfr and/or water reabsorption in superficial relative to deep nephrons.