What is the buffer system in the human body which controls pH inside cells and in the urinary tract.?

The phosphate buffer system operates in the internal fluid of all cells. This buffer system consists of dihydrogen phosphate ions (H2PO4-) as hydrogen-ion donor (acid) and hydrogen phosphate ions (HPO42-) as hydrogen-ion acceptor (base). These two ions are in equilibrium with each other as indicated by the chemical equation below.


H2PO4-(aq) <-------> H+(aq) + HPO42-(aq)

If additional hydrogen ions enter the cellular fluid, they are consumed in the reaction with HPO42-, and the equilibrium shifts to the left. If additional hydroxide ions enter the cellular fluid, they react with H2PO4-, producing HPO42-, and shifting the equilibrium to the right. The equilibrium-constant expression for this equilibrium is

Ka = ([H +] [HPO42-]) /[H2PO4- ]



The value of Ka for this equilibrium is 6.23 脳 10-8 at 25掳C. From this equation, the relationship between the hydrogen-ion concentration and the concentrations of the acid and base can be derived.

[H +] = Ka ([H2PO4-] /[HPO42-] )

Thus, when the concentrations of H2PO4- and HPO42- are the same, the value of the molar concentration of hydrogen ions is equal to the value of the equilibrium constant, and the pH is equal to the pKa (-log Ka), namely 7.21. Buffer solutions are most effective at maintaining a pH near the value of the pKa. In mammals, cellular fluid has a pH in the range 6.9 to 7.4, and the phosphate buffer is effective in maintaining this pH range.