Fig. 1.

Diagram showing the locations of the chief urea transporters (UT-A1, UT-A2, UT-A3), aquaporins (AQP2-4), and sodium co-transporter (NKCC2) involved in urine concentration. The major regions of the kidney are indicated on the right. NaCl is actively reabsorbed across the thick ascending limb by the apical plasma-membrane Na-K-2C1 co-transporter (NKCC2). Water is reabsorbed across the descending limb segments by AQP1 water channels in both apical and basolateral plasma membranes. Water is reabsorbed across the apical plasma membrane of the entire collecting duct by AQP2 water channels when vasopressin is present. Water is reabsorbed across the basolateral plasma membrane by AQP3 water channels in the cortical and outer medullary collecting ducts and by both AQP3 and AQP4 water channels in the IMCD. Urea is concentrated within the collecting-duct lumen (by water reabsorption) until it reaches the terminal IMCD, where it is reabsorbed by the urea transporters UT-A1 and UT-A3.