Normal Acid-Base Mechanisms

pH is related only to [H⁺]. If blood gas analysis indicates that pH is decreased, this is called acidemia; increased pH is referred to as alkalemia. In contrast, the terms acidosis and alkalosis refer to the process that results in acidemia and alkalemia, respectively. Buffer systems exist to minimize changes in pH resulting from H⁺ produced during metabolism. Although many buffer systems exist, the most important for clinical evaluation is the bicarbonate/carbonic acid system:

The kidneys (metabolic system) can control HCO₃^– by excretion into or reabsorption from the urine, as well as generation of new bicarbonate. Increases in bicarbonate will shift the equilibrium to the right (to carbonic acid formation) thereby resulting in decreased H⁺. Decreases in bicarbonate result in a shift to the left and increased H⁺. This compensatory response occurs relatively slowly over a few days.

The lungs (respiratory system) can control CO₂ by hypoventilation or hyperventilation. Increased CO₂ (hypoventilation) will shift the equilibrium to the left and result in increased H⁺ and acidosis. Decreased CO₂ (hyperventilation) will shift the equilibrium to the right and cause decreased H⁺ and alkalosis.