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Fig. 4. Transfer of NH4+ from the loop of Henle (LOH) to the medullary collecting duct (MCD). The medullary thick ascending limb (mTAL) of the LOH is shown on the far left and the MCD is shown on the right side of the figure. Reabsorption of NH4+ from the mTAL adds NH3 to the interstitial compartment (the H+ to convert it to NH4+ arrives at site 4). Recycling of NH4+ in the LOH raise the concentration of NH4+ in the medullary interstitium (site 1). NH4+ diffuses through the renal medullary interstitial compartment because its concentration is high and that of NH3 is too low for rapid rates of diffusion (site 2). NH4+ crosses the cell membranes of the MCD using two different NH4+/H+ exchangers, one on each of these cells (sites 3 and 4). The combination of NH4+ entry into and H+ exit from the lumen of the MCD (site 4) adjusts the urine pH upward (towards 6.0) despite continuing H+ secretion by the H+-ATPase (site 5). The net result is a final urine pH that is close to 6.0 and a somewhat higher rate of NH4+ excretion.





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