Box 24 Biochemistry In Medicine

Blood, Lungs, and Buffer: The Bicarbonate Buffer System

In animals with lungs, the bicarbonate buffer system is an effective physiological buffer near pH 7.4, because the H2CO3 of blood plasma is in equilibrium with a large reserve capacity of CO2(g) in the air space of the lungs. This buffer system involves three reversible equilibria between gaseous CO2 in the lungs and bicarbonate (HCOjT) in the blood plasma (Fig. 1).

When H+ (from lactic acid produced in muscle tissue during vigorous exercise, for example) is added to blood as it passes through the tissues, reaction 1 proceeds toward a new equilibrium, in which the concentration of H2CO3 is increased. This increases the concentration of CO2(d) in the blood plasma (reaction 2) and thus increases the pressure of CO2(g) in the air space of the lungs (reaction 3); the extra CO2 is exhaled. Conversely, when the pH of blood plasma is raised (by NH3 production during protein catabo-lism, for example), the opposite events occur: the H+ concentration of blood plasma is lowered, causing more H2CO3 to dissociate into H+ and HCOjT. This in turn causes more CO2(g) from the lungs to dissolve in the blood plasma. The rate of breathing—that is, the rate of inhaling and exhaling CO2—can quickly adjust these equilibria to keep the blood pH nearly constant.

Aqueous phase H2 (blood in capillaries)

Diabetes 2

Diabetes 2

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