Proximal sodium and fluid transport

Renal salt transport constitutes the main driving force for transepithelial water movement and thus assumes a key role in maintaining water balance. Most nephron segments are characterized by primary transport of sodium ions, although in a restricted portion of the nephron, the ascending limb of Henle's loop, chloride transport [1–4] is primary. It is well established that the bulk of filtered salt and water is reabsorbed by the proximal tubule; on the other hand, it has been well-known for decades that the fine adjustment of salt and water balance occurs in the distal portions of the nephron, i.e., the distal convoluted tubule and the collecting ducts. Under certain conditions proximal tubular function can dictate salt and water balance in the body, for example, during strong inhibition of proximal tubular transport when the distal nephron is overloaded [5]. In addition, it appears likely today from studies on renal ephithelium as well as other epithelial structures [6] that sodium transport may play an important role in restoring filtered nonelectrolytes to the body by coupled co-transport mechanisms.