Circulation during hypoxia in birds.

The effects of hypoxia on the avian cardiovascular system are reviewed. The avian cardiovascular system seems well adapted to deal with the stress of hypoxia. In general, birds are remarkably tolerant of hypoxia, with some species being capable of performing vigorous exercise at extreme altitude. During hypoxia at rest, the circulation maintains arterial pressure, increases cardiac output, and redistributes blood flow so oxygen delivery to the heart and brain is maintained. During exercise, further adjustments are required, since exercising muscle has large oxygen requirements. The mechanisms responsible for producing these circulatory changes are largely unknown. The transport steps that limit O2 delivery during hypoxia are also poorly understood.