There are two types of acid-base disorders: acidosis and alkalosis. These disorders can be either metabolic (decreased or increased HCO3–) or respiratory (decreased or increased PCO2). Primary respiratory disorders affect blood acidity by changes in PCO2, and primary metabolic disorders are disturbances in HCO3– concentration. A primary disturbance is usually accompanied by a compensatory response, but the compensation does not fully correct the pH disturbance of the primary disorder. If the pH is < 7.40, the primary process is acidosis, either respiratory (PCO2 greater than 40 mm Hg) or metabolic (HCO3– less than 24 mEq/L). If the pH is > 7.40, the primary process is alkalosis, either respiratory (PCO2 less than 40 mm Hg) or metabolic (HCO3– greater than 24 mEq/L). One respiratory or metabolic disorder with its appropriate compensatory response is a simple acid-base disorder.