Altitude sickness is a pathological effect of high altitude on humans, caused by acute exposure to low partial pressure of oxygen at high altitude. It commonly occurs above 2,400 metres (8,000 feet). It presents as a collection of nonspecific symptoms, acquired at high altitude or in low air pressure, resembling a case of "flu, carbon monoxide poisoning, or a hangover". Being exposed to low atmospheric oxygen results in hyperventilation and respiratory alkalosis. The body compensates metabolically by increasing the amount of erythropoietin, resulting in increased hematocrit and hemoglobin. Chronic hypoxic pulmonary vasoconstriction results in pulmonary hypertension and right heart failure.