Impact of Rat Outer Medullary Architecture on Oxygen Distribution

Abstract

In this study, we examined the impact of the complex structural organization of the rat outer medulla (OM) on oxygen distribution. We extended a previous region‐based mathematical model of the urine concentrating mechanism in the rat OM to incorporate the transport of red blood cells, hemoglobin, and O2. We considered O2 consumption for basal cellular metabolism and for active transport of NaCl across medullary thick ascending limbs (mTALs). Our model predicts that the OM structural organization results in significant oxygen tension (PO2) gradients both in the axial and the radial directions. The segregation of descending vasa recta, the main supply of O2, at the center and immediate periphery of the vascular bundles gives rise to large radial differences in PO2 between regions and helps preserve O2 delivery to the inner medulla. Assuming that the mTAL active Na+ transport rate decreases when mTAL PO2 falls below a critical level, oxygen availability to mTALs has a significant impact on the concentrating capability of the model OM. The model also predicts that when the OM undergoes hypertrophy, its concentrating capability increases significantly only when anaerobic metabolism supports a substantial fraction of the mTAL active Na+ transport, which is otherwise critically reduced by the low interstitial and mTAL luminal PO2 in a hypertrophied OM.Support: NIH Grant DK‐53775 (AE and AL) and NSF Grant DMS‐0701412 (AL).