High thermal sensitivity of blood enhances oxygen delivery in the high-flying bar-headed goose

The bar-headed goose (Anser indicus) crosses the Himalaya twice a year at altitudes where oxygen (O-2) levels are less than half those at sea level and temperatures are below -20 degrees C. Although it has been known for over three decades that the major hemoglobin (Hb) component of bar-headed geese has an increased affinity for O-2, enhancing O-2 uptake, the effects of temperature and interactions between temperature and pH on bar-headed goose Hb-O-2 affinity have not previously been determined. An increase in breathing of the hypoxic and extremely cold air experienced by a bar-headed goose at altitude (due to the enhanced hypoxic ventilatory response in this species) could result in both reduced temperature and reduced levels of CO2 at the blood-gas interface in the lungs, enhancing O-2 loading. In addition, given the strenuous nature of flapping flight, particularly in thin air, blood leaving the exercising muscle should be warm and acidotic, facilitating O-2 unloading. To explore the possibility that features of blood biochemistry in this species could further enhance O-2 delivery, we determined the P-50 (the partial pressure of O-2 at which Hb is 50% saturated) of whole blood from bar-headed geese under conditions of varying temperature and [CO2]. We found that blood-O-2 affinity was highly temperature sensitive in bar-headed geese compared with other birds and mammals. Based on our analysis, temperature and pH effects acting on blood-O-2 affinity (cold alkalotic lungs and warm acidotic muscle) could increase O-2 delivery by twofold during sustained flapping flight at high altitudes compared with what would be delivered by blood at constant temperature and pH.