Earliest change in high altitude is
Immediate exposure to low PO2 stimulates the aerial chemoreceptors, and this stimulation increases alveolar ventilation to a maximum of about 1.65 times normal. Therefore, compensation occurs within seconds for the high altitude, and it alone allows the person to rise several thousand feet higher than would be possible without the increased ventilationGuyton 13 e pg: 563
Hyperventilation
Decrease in work capacity
Drowsiness
Polycythemia
Respiratory system