Effects of anastomoses on solute transcapillary exchange in countercurrent systems.

OBJECTIVE

To investigate effects of anastomoses between the descending vasa recta (DVR) and the ascending vasa recta (AVR) on the distribution of small solutes in the interstitial fluid of the renal medulla.

METHODS

Countercurrent capillary loops, surrounded by a secretory epithelium, were used to model microvessels in the renal medulla. Anastomoses between the DVR and AVR were modeled as a decrease in cross-sectional area of the vessels and a decrease in flow velocity from the base to the tip of the capillary loop. When experimental data were used to evaluate parameters of the model, it was seen that diffusive transport of solute in the axial direction of the capillary was negligible, and the equations could be greatly simplified.

RESULTS

General formulae of the solute concentration distribution were derived for different degrees of shunting between the two limbs of the capillary loop. Analytical solutions for the steady-state solute distribution were obtained when the sizes of capillaries and flows in them were assumed to decrease linearly with the distance from the base to the tip of the capillary loop. When the effects of reduction in the size of the limbs were compared with the effects of reduction of flow velocities on solute distribution, it was found that, in the presence of anastomoses, change in flow velocity increases the axial gradient of the solute concentration more than change in the cross-sectional area. The combined effects of a decrease in flow velocity and cross-sectional area can easily double the axial gradient of the solute concentration for a modest degree of anastomotic shunting.

CONCLUSIONS

In this study, we separated effects of anastomotic flow between the DVR and AVR from other factors affecting the complicated countercurrent solute exchanges in the renal medulla. Results from the model show that anastomoses increase the solute concentration in the medullary interstitium and also the axial gradient of the solute concentration there.