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The source of the secreted Post Date: Thu, 16 Oct 2008 22:44:11 +0000 Under certain circumstances the amount of titratable acid in the voided urine cannot be accounted for by the selective reabsorption of constituents of the filtrate. This provides proof that hydrogen ion is added to or secreted into the tubular fluid by the tubular cells. The source of the secreted hydrogen ions is carbonic acid derived from the hydration of carbon dioxide. Autor of the post: Undefined This applies to normal conditions Post Date: Thu, 16 Oct 2008 22:25:47 +0000 Present evidence strongly suggests that the primary event is the secretion of H+ itself, rather than the reabsorption of anion (HC03~).It has long been known that the kidney plays an important role in maintaining acid-base balance. This applies to normal conditions but increases in importance as a homeostatic compensation to metabolic acidosis. Autor of the post: Undefined In the process of bicarbonate reabsorption Post Date: Thu, 16 Oct 2008 22:07:10 +0000 The renal response can be described in four separate parameters, as follows: (1) the complete reabsorption of filtered sodium bicarbonate, (2) the acidification of the urinary buffers (ie, the production of titratable acid), (3) the excretion of fixed anions in combination with NH4+ rather than Na+, and (4) the adjustment of urinary pH or H+ ion concentration. Each process can be considered in terms of the same underlying mechanism.In the process of bicarbonate reabsorption, the H+ derived from H+HC03~ in the tubular cell is exchanged with Na+ in the tubular urine. Autor of the post: Undefined This C02 then readily back-diffuses Post Date: Thu, 16 Oct 2008 21:54:49 +0000 The Na+ combines with the HC03~ in the tubular cell and is returned to the extracellular fluid as Na+HC03 . The H+ in the tubular urine combines with HC03" to form H2C03, which is rapidly broken down to C02 and H20. This C02 then readily back-diffuses across the tubular epithelium to become admixed with the carbonic acid-bicarbonate pool of the body. Autor of the post: Undefined Under these circumstances, H+ will Post Date: Thu, 16 Oct 2008 21:43:09 +0000 The overall reaction is the reabsorption of Na+ and HC03".When all the bicarbonate in the tubular urine has been removed, the mechanism of H+-Na+ exchange can still continue. Under these circumstances, H+ will be added to the buffer systems in the urine, primarily phosphate, with the conversion of HP042~ to H2P04". Autor of the post: Undefined On the other hand Post Date: Thu, 16 Oct 2008 21:26:57 +0000 The exchanged Na+ will be returned to the extracellular fluid as Na+HC03~ and thus contribute to the available fixed cation of the extracellular fluid. If Na+-H+ exchange proceeds at a rate insufficient to reabsorb the filtered bicarbonate, an alkaline urine containing large amounts of bicarbonate will be excreted. On the other hand, at maximal rates of H+ transport, not only will all the bicarbonate disappear from the urine but also the titratable acidity of the urine will rise as a result of acidification of the buffer systems and more Na+HC03~ will be returned to extracellular fluid than was filtered at the glomerulus. Autor of the post: Undefined (The minimal pH that can Post Date: Thu, 16 Oct 2008 21:09:24 +0000 If HC03 and phosphate buffers were not present in the tubular urine, Na+-H+ exchange would involve the cation of a neutral salt (e g, Na+Cl~) and would result in formation of a strong acid. This would increase the hydrogen ion concentration of the urine to such an extent that further transport of H+ would be blocked because of the concentration gradient of H+ between tubular cell and tubular urine. (The minimal pH that can be achieved in the urine of man is 4,4 to 4,5. Autor of the post: Undefined If the urine is acidic Post Date: Thu, 16 Oct 2008 20:50:24 +0000 ) However, in response to the need for conservation of fixed cation,the kidney synthesizes ammonia. When ammonia is formed in the renal tubular cells, it diffuses readily into the tubular urine. If the urine is acidic as a result of Na+-H+ exchange, the ammonia that diffuses immediately reacts with H+ to form NH4+. Autor of the post: Undefined In short, the two processes Post Date: Thu, 16 Oct 2008 20:36:07 +0000 The renal tubule is impermeable to the charged particle (NH4+), and hence it does not diffuse back out of the tubular fluid. There are two important consequences of this reaction: first, H+ is removed, and this permits further Na+-H+ exchange to occur; second, NH3 is removed, and this permits more NH3 to diffuse from tubular cell to tubular urine. In short, the two processes occur concurrently and can continue only by aiding and abetting each other. Autor of the post: Undefined This catalyzes both the hydration Post Date: Thu, 16 Oct 2008 20:18:43 +0000 By this sequence of events, large amounts of Na+ can be retrieved from neutral salts and returned to the extracellular fluid as sodium bicarbonate (2 in Figure VIII-1).The above discussion is of pharmacological significance because the rate of Na+-H+ exchange in the renal tubule can be greatly decreased by drugs that inhibit the enzyme carbonic anhydrase. This catalyzes both the hydration of C02 and dehydration of H2C03 and thus determines the relative concentrations of these molecular species. 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