Mathematical model of nitric oxide convection and diffusion in a renal medullary vas rectum.

In this study, the generation, convection, diffusion, and consumption of nitric oxide (NO) in and around a single renal medullary descending or ascending vas rectum in rat were modeled using CFD. The vascular lumen (with a core RBC-rich layer and a parietal layer), the endothelium, the pericytes and the interstitium were represented as concentric cylinders. We accounted for the generation of NO by vascular endothelial cells, and that by the epithelial cells of medullary thick ascending limbs (mTALs) and inner medullary collecting ducts (IMCDs), the latter via interstitial boundary conditions. Luminal velocity profiles were obtained by modeling blood flow dynamics. Our results suggest that convection (i.e., blood flow per se) does not significantly affect NO concentrations along the cortico-medullary axis, because the latter are mostly determined by the rate of NO production and that of NO consumption by hemoglobin. However, the shear stress-mediated effects of blood flow on NO generation rates, and therefore NO concentrations, were predicted to be important. Finally, we found that unless epithelial NO generation rates (per unit tubular surface area) are at least 10 times lower than endothelium NO generation rates, NO production by mTALs and IMCDs affects vascular NO concentrations, with possible consequences for medullary blood flow distribution.