Cardiovascular and respiratory responses of ducks to progressive hypocapnic hypoxia.

A fall in oxygen tension of the inspired air demands that both circulatory and respiratory adjustments be made if the tissues are to receive an adequate supply of oxygen. Increase in ventilation will reduce the oxygen tension difference between the air breathed and the arterial blood, but even so when the oxygen tension of the arterial blood (PB> 0 | ) falls a decrease in oxygen supply to the tissues will ensue unless there are adjustments in the circulatory compartment. These do not necessarily entail an increase in cardiac output; for when exposed to hypocapnic hypoxia (altitude) a pronounced Bohr shift coupled with increased oxygen extraction by the tissues, reflected in a lowering of Pv_ Ot, could go some way towards achieving the same end. Ducks certainly show large increases in respiratory minute volume in response to hypoxia (Jones & Purves, 1970) but there is no information concerning any changes in cardiac output or oxygen extraction by the tissues. Butler (1970) reports a significant tachycardia in ducks only whenPa Oi falls below 35 mmHg, which implies an increase in cardiac output at this level of hypoxia. Since the blood of many flying birds shows a low affinity for oxygen and a pronounced Bohr shift (Lenfant, Kooyman, Eisner & Drabek, 1969), during exposure to high altitude marked changes in cardiac output may not become necessary for maintenance of oxygen supply to the tissues until Pa> o> is very low. The present paper concerns itself with measuring the respiratory and cardiac responses of resting unanaesthetized ducks to simulated high altitudes, and includes measurements of gaseous exchange at both tissues and lungs under these conditions.