Theoretical effects of a distribution of capillary dimensions on glomerular ultrafiltration

Simple network models were developed to examine the effects of a distribution of capillary dimensions on the filtration of fluid in the renal glomerulus. Two idealized networks were considered, one with parallel capillaries of varying length, and the other with parallel capillaries of varying radius. It was found that for a fixed value of the ultrafiltration coefficient (Kf), the product of hydraulic permeability and total capillary surface area per glomerulus, any assumed heterogeneity of capillary dimensions lowered the single nephron glomerular filtration rate (SNGFR) below that predicted for uniform capillaries in parallel. This implies that, to the extent that the capillaries are not identical, a given filtering surface area is used less efficiently. A consequence of this is that for a given set of measured glomerular pressures and flows, a model which assumes identical capillaries will underestimate the value of Kf for the actual (heterogeneous) glomerular network. The effects of a distribution in capillary lengths were found to be more pronounced, in general, than those of a distribution in radii. Considering only heterogeneity in capillary length, and characterizing the effective length distribution using available data in the literature, it was found that calculating Kf using the customary assumption of identical capillaries will underestimate the true value by approximately 30%, under typical micropuncture conditions in the rat. The predicted dependence of SNGFR on afferent plasma flow rate, transmural hydraulic pressure difference, and afferent protein concentration was quite similar, however, whether capillary dimensions were assumed to be uniform or heterogeneous.