| Abstract: | The literature on hydronatriuresis control processes operating at the level of individual renal functional units and of the organ as a whole is analysed. 1) Elementary sodium salt and water tubular transport mechanisms. In converting the filtrate into urine, the kidney expends metabolic energy: this is used in the (active) transport of sodium salts; (passive) transport of water takes place along the osmotic gradients created by salt transfer. The proximal tubules reabsorb the sodium bic-rbonate actively. The reabsorption of the osmitic equivalent of water has the effect of concentrating NaCl in the tubular fluid. An important role in the reabsorption of NaCl is played by passive diffusion from the lumen to the interstitial fluid; the remainder is transferred actively, perhaps by an electrically neutral pump. With respect to the other nephronic segments, the proximal tubule has a relatively high passive permeability to water and salts: active transport here must not surmount high friction resistances nor take place against important concentration gradients. The low permeability of the distal nephron, on the other hand, increases the energy cost of salt transport; for the same reason, important electrochemical gradients are created and the composition of tubular fluid is drastically altered. 2) Elementary mechanisms of tubular potassium transport. Potassium is reabsorbed actively along the whole nephron by a luminal pump. The proximal tubules and Henle loops promote practically complete absorption of filtrated potassium. The distal tubules and collectors have the two-fold capacity of secreting and reabsorbing cation: the quantity of potassium excreted with the urine depends on the degree of excess of the secretion process. At distal tubular level, potassium secretion is a passive phenomenon dependent on the favourable transluminal gradient of the cation's electrochemical potential. 3) Renal function and volume homoeostasis of extracellular fluid. The organism's sodium content is largely controlled by renal excretion of sodium; homoeostasis of the sodium mass guarantees volume homoeostasis of the extracellular fluid through thirst and osmotic secretion of ADH. Extracellular fluid volume errors are picked up by the organism to the extent to which they translate themselves into pressure variations in the low pressure vascular system or into variations in haematic constituent concentration within the vascular sector, produced with velocities independent, at least in the short term, of the volume of extracellular fluid. In control of natriuria are the glomerular filtrate, intrarenal distribution of blood flow and tubular reabsorption of sodium; in its turn, the latter is subject to nervous and hormonal influences and influences from the physical environment surrounding the nephrons... |
