All the water-soluble waste products and all the ions and water ingested in excess of needs must be excreted in the urine without forming precipitates. One of the most important renal physiological functions is to excrete urine with a composition that makes ions and organic materials sufficiently soluble to avoid kidney stone formation. Key to this aim is to have independent regulation of the urine pH -select a value that is close to 6.0 to achieve this aim  (Figure 10). Excreting urine at
Figure 10. UrinepHandkid-ney stones. The safest urine pH to avoid kidney stones is close to 6. Be I ow this value, uric acid stones are most likely to form. Ca-phosphate stones precipitate in alkaline urine. By driving NH4+ excretion with a high distal H+ secretion, a considerable quantity of H+ can be elimi nated at a urine pH close to 6.0. By excreting organic anions rather than HCO3", a considerable quantity of HCO3-can also be eliminated at a urine pH close to 6.0 (see Figure 2).
this pH must not sacrifice acid-base balance. This in turn means that with a large, chronic acid load, the excreiion of NH4+ should be maximally high at a urine pH of close to 6 (see reference Kamel et al. 1998  formore discussion). Similarly, when an alkali load is ingested, it must be excreted without obligating a large excretion of HCO3- and thereby a high urine pH [9, 11] - this latter topic will be discussed in more deiail when CaHPO4 stones are considered below.
Avoiding uric acid kidney stones: Uric acid is the waste product of purine meiabo-lism . The free acid form, uric acid, rather than total urates is the critical component for kidney stone formation because uric acid is sparingly soluble in water (Equation 4). Because the pK of uric acid in the urine at 3 7°C is close to 5.3 , precipitation of uric acid can be avoided without increasing the urine volume by raising the urine pH to 6 at the same to tal urate excretion rate .
Avoiding CaHPO4 kidney stones: All the ionized Ca2+ absorbed from the GI tract of an adult is excreted in the urine in steady state
. The chemistry of Ca2+ and HPO4 in the urine is the same as in the body , but with one ex cep tion. A met abolic equiv alent of
HCO3- in acid-base terms, ciirate , can chelate ionized Ca + in the urine, forming a soluble ion complex and thereby minimize the risk of CaHPO4 kidney stone formation.
This excretion of ciirate rises with an alkali load , a time when urine H2PO4- is con-2-
verted to HPO4 " - i.e., when the urine pH rises towards the pK of the phosphate buffer system (pH 6.8). In this context, the body disposes of the usual alkali load of the diet by forming an organic acid such as citric acid [9, 11]. The H+ of citric acid titrate HCO3". The citrate anions rather than an appreciable amount of HCO3" are excreted in the urine so that the urine pH remains close to 6.0 without sacrificing acid-base balance  (Figure
10). Excreting citrate rather than HCO3- when
there is a dietary alkali load chelates Ca in the urine, lessening the risk of Ca-containing kidney stones in alkaline urine .
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