Intrarenal distribution of blood flow

Renal blood flow is mostly directed to the cortex to optimize glomerular filtration and bulk solute reabsorption. In contrast, blood flow to the renal medulla is low to preserve osmotic gradients and to enhance urinary concentration (Fig 1). Within the medulla, tubules and vasa recta are disposed in a hairpin pattern to maximize urine concentration by countercurrent exchange. Oxygen diffuses from arterial to venous vasa recta, leaving the outer medulla deficient in oxygen. In this region, the medullary thick ascending limb is responsible for the generation of an osmotic gradient by active NaCl reabsorption, a process that has high oxygen requirements. Medullary hypoxia under normal conditions has been documented using oxygen microelectrodes in several mammalian species including humans ( Brezis.§nd.Ro§®B 1995). Renal medullary hypoxia is an obligatory price that the mammalian kidney pays for successful urinary concentration.

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