The second major category of acidosis is that called respiratory acidosis. This may be due to any condition that causes pulmonary CO2 retention. These conditions include the respiratory muscle paralysis of poliomyelitis, the respiratory brain center depression sometimes seen in encephalitis or with large doses of such drugs as morphine, primary lung disease (e.g., pulmonary fibrosis or severe emphysema) that destroys oxygen exchange ability, and sometimes heart diseases (e.g., chronic congestive heart failure). The basic problem is H2CO3 excess produced by CO2 retention. Thus, the denominator of the Henderson-Hasselbalch equation is increased, the normal 20:1 ratio is decreased, and the pH is decreased. The CO2 content is sometimes normal but is usually increased, because of kidney attempts to handle the excess CO2 by forming more HCO–3 and excreting more H+ ions.