Phosphate buffering system: regulating acid-base balance in the body
The phosphate buffering system has a pK of 6.8, which is not far from normal pH in body fluids of 7.4; This allows the buffering system to work close to maximum.
Although the phosphate buffering system is not as important as an extracellular fluid buffering system, it plays an important role in renal tubular buffer and intracellular fluids.
The main elements of the phosphate buffering system are H2PO4- and HPO4 2-. When a strong acid like HCl is added to a mixture of these two substances, the hydrogen is accepted by the HPO4 2- base and converted to H2PO4-.
HCl + Na2HPO4 = > NaH2PO4 + NaCl
The result of this reaction is that the strong acid HCl, is replaced by a weak acid, NaH2PO4, and the pH is minimized. When a strong base, such as NaOH, is added to the buffering system, OH- is buffered by H2PO4- to form HPO4 + H2O.
NaOH + NaH2PO4 = > Na2HPO4 + H2O
In this case, a strong base, NaOH, is being replaced by a weak base, Na2HPO4, which only increases the pH slightly.
The phosphate buffering system has a pK of 6.8, which is not far from normal pH in body fluids of 7.4; This allows the buffering system to work close to maximum. However, its concentration in the extracellular fluid is low, only about 8% of the bicarbonate buffer. Therefore, the total number of electrical buffers of the phosphate system in the extracellular fluid is much less than that of the systemic bicarbonate buffer.
In contrast to its small role as an extracellular buffer, the phosphate buffer is particularly important in the tubular formation of kidney secretions for two reasons: (1) phosphate often becomes highly concentrated in the renal tubules. thus, increasing the buffering strength of the phosphate system, and (2) the in-tube solutions typically have a pH significantly lower than the extracellular fluid pH, making the buffer pH close to the pK (6.8) of the system.
The phosphate buffering system is also important in the intracellular fluid buffer because the phosphate concentration in this fluid is higher than that of the extracellular fluid. In addition, the pH of the intracellular fluid is lower than that of the extracellular fluid and is therefore usually closer to the pK of the phosphate buffering system than the extracellular fluid.