Effects of pH and medullary blood flow on oxygen transport and sodium reabsorption in the rat outer medulla

Abstract

We used a mathematical model of O2transport and the urine concentrating mechanism of the outer medulla of the rat kidney to study the effects of blood pH and medullary blood flow on O2availability and Na+reabsorption. The model predicts that in vivo paracellular Na+fluxes across medullary thick ascending limbs (mTALs) are small relative to transcellular Na+fluxes and that paracellular fluxes favor Na+reabsorption from the lumen along most of the mTAL segments. In addition, model results suggest that blood pH has a significant impact on O2transport and Na+reabsorption owing to the Bohr effect, according to which a lower pH reduces the binding affinity of hemoglobin for O2. Thus our model predicts that the presumed greater acidity of blood in the interbundle regions, where mTALs are located, relative to that in the vascular bundles, facilitates the delivery of O2to support the high metabolic requirements of the mTALs and raises the concentrating capability of the outer medulla. Model results also suggest that increases in vascular and tubular flow rates result in disproportional, smaller increases in active O2consumption and mTAL active Na+transport, despite the higher delivery of O2and Na+. That is, at a sufficiently high medullary O2supply, O2demand in the outer medulla does not adjust precisely to changes in O2delivery.