The phosphate buffer system is composed of HPO4= and H2PO4-. Both become concentrated in the tubular fluid because of their relatively poor reabsorption and because of the reabsorption of water from the tubular fluid. Therefore, although phosphate is not an important extracellular fluid buffer, it is much more effective as a buffer in the tubular fluid.
Another factor that makes phosphate important as a tubular buffer is the fact that the pK of this system is about 6.8. Under normal conditions, the urine is slightly acidic, and the urine pH is near the pK of the phosphate buffer system. Therefore, in the tubules, the phosphate buffer system normally functions near its most effective range of pH.
Figure 30-7 shows the sequence of events by which H+ is excreted in combination with phosphate buffer and the mechanism by which new bicarbonate is added to the blood. The process of H+ secretion into the tubules is the same as described earlier. As long as there is excess HCO3- in the tubular fluid, most of the secreted H+ combines with HCO3-. However, once all the HCO3- has been reabsorbed and is no longer available to combine with H+, any excess H+ can combine with HPO4= and other tubular buffers. After the H+ combines with HPO4= to form H2PO4-, it can be excreted as a sodium salt (NaH2PO4), carrying with it the excess hydrogen.
There is one important difference in this sequence of H+ excretion from that discussed previously. In this case, the HCO3- that is generated in the tubular cell and enters the peritubular blood represents a net gain of HCO3- by the blood, rather than merely a replacement of filtered HCO3-. Therefore, whenever an H+ secreted into the tubular lumen combines with a buffer other than HCOi, the net effect is addition of a new
HCO3 to the blood. This demonstrates one of the mechanisms by which the kidneys are able to replenish the extracellular fluid stores of HCO3-.
Under normal conditions, much of the filtered phosphate is reabsorbed, and only about 30 to 40 mEq/day is available for buffering H+. Therefore, much of the buffering of excess H+ in the tubular fluid in acidosis occurs through the ammonia buffer system.
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