ABSTRACT
Initial responses On exposure to high altitude, lowlanders hyperventilate (up to 50% above the sea-level rate over the first 7 days of exposure). Later, there is some compensation (also seen in native Andean highlanders) through deeper breathing. Hyperventilation increases arterial oxygen levels but also reduces blood carbon dioxide levels and raises the proportion of oxygen in the alveolar gas mixture (since the alveolar gases are more or less in equilibrium with the pulmonary blood gases). Thus the pO2 gradient between inspired air and the gas mixture in the alveoli is reduced, and a sufficiently high pO2 level is maintained in the alveoli to allow oxygen diffusion into the blood. The reduction in blood pCO2 causes respiratory alkalosis, corrected by increased kidney excretion of HCO3-. Before this compensation occurs, alkalosis can inhibit the respiratory center of the brain leading to hypoxia (shortage of oxygen), characterized by cyanosis (blueing of the extremities such as the lips and ear lobes due to deoxygenated arterial blood) and impaired mental performance, such as judgement and concentration. The cardiac rate increases transiently to improve oxygen transport to the tissues.
