An overview of salt absorption by the nephron

The purpose of this brief commentary is to provide a description of how renal physiology, and more particularly, renal tubular physiology, has evolved over the past thirty years, since the occasion, obviously, is a celebration of the Thirtieth Course on Advances in Nephrology and Dialysis. My arguments will begin by quoting from Homer Smith's book, The Kidney, and merging Smith's observations, which were integrative in the sense that they did not specify detail, with detailed incursions into tubular physiology, first at a cellular level and then at a molecular level. For convenience, the nephron is divided into four functional segments: the proximal nephron; the ascending limb; the distal nephron; and the collecting duct. Each of these carries out a specific function. The proximal nephron absorbs about two-thirds of filtered sodium, without dissociating salt and water absorption. The thick ascending limb absorbs 25% of filtered Na+, but no water. The distal nephron absorbs 10% of filtered Na+ in close relation with K+ and, to some extent, H+ secretion. Finally, the collecting duct includes three kinds of cells: the cortical collecting duct, which is responsible not only for Na+ absorption and K+ secretion, but also for the bulk of the absorption of free water; the outer medullary collecting duct (OMCD), which is largely responsible for the final steep drop in urine pH which occurs between cortex and papilla; and the inner medullary collecting duct (IMCD), whose major function is the final absorption of approximately 5% of filtered Na+.