Mechanism of Concentration and Dilution of Urine

The chapter begins with an overview, showing how the regulation of water permeability of the distal nephron, combined with the osmotic gradient established in the kidney interstitium, allows the kidney to concentrate or dilute the urine. It then shows how the kidney establishes this osmotic gradient by using the counter-current flow of the loop of Henle and collecting ducts, and the spatial separation of transport mechanisms, to make a gradient greater than that achievable by the transport mechanisms themselves. The essential role of the vasa recta is discussed. The contribution of urea to the osmotic gradient in the inner medulla is explained. The function of ADH in allowing the contribution of urea is also discussed. The mechanism of osmotic diuresis is explained.