UREA TRANSPORT AND THE URINARY CONCENTRATING MECHANISM

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Studies of isolated renal epithelia and mathematical modelling studies are being carried out to investigate the role of urea in the concentrating mechanism and to define the mechanism of concentration in the inner medulla. The inner medullary collecting duct was found to be heterogenous with respect to urfea permeability. Collecting ducts from the outer third of the inner medulla have a low permeability to urea like those from the medullary rays and outer medulla. In contrast, collecting ducts from the inner two-thirds of the inner medullary collecting duct have an extremely high permeability to urea, 40 x 10-5 cm/s. This pattern of collecting duct urea permeabilities maximizes the axial urea gradient in the inner medulla by delaying urea absorption to near the papillary tip where the effective blood flow is lowest. The papillary surface epithelium was found to have a low permeability to urea (1 x 10-5 cm/s) and a small surface area relative to inner medullary collecting ducts. It is concluded that urea transport across the papillary epithelium is unlikely to contribute significantly to urea delivery to the inner medullary interstitium.