The other major subdivision of alkalosis is respiratory alkalosis, which occurs when the respiratory mechanism blows off more CO2 than it normally would due to respiratory center stimulation from some cause. The main conditions in which this happens are the hyperventilation syndrome caused by hysteria or anxiety, high fever, and direct stimulation of the respiratory center by drugs. Overdose of aspirin can cause respiratory alkalosis in the early stages; although later, after more of the aspirin is absorbed, a metabolic acidosis develops. In hyperventilation of whatever cause, respirations are increased and deeper, blowing off more CO2. This creates an H2CO3 deficit since it is being used up to replenish CO2 by the lung carbonic anhydrase enzymes. Therefore, the denominator of the Henderson-Hasselbalch equation is decreased, the 20:1 ratio is increased, and plasma pH increased. The CO2 content will decrease, because when H2CO3 is lost due to formation of CO2 in the lungs, HCO3– is converted to H2CO3 in the kidney to compensate secondarily for or to replace the decreasing plasma carbonic acid.