Some animals have adapted to hypoxia by increasing their haemoglobin affinity for oxygen, but in vitro studies have not shown any change of haemoglobin affinity for oxygen in human high altitude natives or lowlanders acutely acclimatized to high altitude. We conducted the first in vivo study of the oxyhaemoglobin dissociation curve by progressively reducing arterial PO2 while maintaining normocapnia in lowlanders at sea level, lowlanders sojourning at 3600m for two weeks and native Andeans at the same altitude. We found that the in vivo PO2 at which haemoglobin is half-saturated (P50) is higher in lowlanders at sea level (32 mmHg) than that measured in vitro (27 mmHg) and that lowlanders and highlanders do significantly increase the in vivo affinity of their haemoglobin for oxygen with exposure to high altitude. These results indicate the value of an in vivo approach for studying the oxyhaemoglobin dissociation curve.

Some animals have adapted to hypoxia by increasing their haemoglobin affinity for oxygen, but in vitro studies have not shown any change of haemoglobin affinity for oxygen in human high altitude natives or lowlanders acutely acclimatized to high altitude. We conducted the first in vivo study of the oxyhaemoglobin dissociation curve by progressively reducing arterial PO2 while maintaining normocapnia in lowlanders at sea level, lowlanders sojourning at 3600m for two weeks and native Andeans at the same altitude. We found that the in vivo PO2 at which haemoglobin is half-saturated (P50) is higher in lowlanders at sea level (32 mmHg) than that measured in vitro (27 mmHg) and that lowlanders and highlanders do significantly increase the in vivo affinity of their haemoglobin for oxygen with exposure to high altitude. These results indicate the value of an in vivo approach for studying the oxyhaemoglobin dissociation curve.