Fig. 1. Urea excretion and the avoidance of oliguria in a subject with a normal protein intake and a deficit of Na⁺, Cl^– and water. The barrel-shaped structure represents the inner MCD; AQP-2 is shown as a clear oval and the urea transporter is shown as a shaded oval in its luminal membrane. The issues in the conundrum are shown to the left of the vertical broken line and features for its resolution are shown to the right of this line. To resolve the conundrum, the hypothesis is that urea can become an effective urine osmole in urine with a low ionic strength. This view is supported by the fact that the concentration of urea is higher in the luminal urine than the interstitial compartment (papilla; Table 2). VP, vasopressin; MCD, medullary collecting duct.