Figure 1.

Function of the renal tubule. (A) Filtration of blood through the glomerulus. Filtrate composition is identical to plasma. (B) Bulk transport in the proximal tubule. Transepithelial flux of solutes and water is high. Tubule fluid composition is moderately different from plasma and transepithelial gradients are just beginning to be established. (C) Fine-tuning in the distal tubule. Transepithelial transport capacity is low but largely driven uphill against steep and thermodynamically unfavorable transepithelial gradients, thereby requiring considerable energy consumption for active transport.