Cerebral blood flow in birds: effect of hypoxia.

The effect of hypoxia on cerebral blood flow in ducks was investigated by the rate at which arterially injected xenon-133 was cleared from the duck's brain. A two-component clearance curve resulted, which we have attributed to flow through the grey and white matter. Decreasing the arterial oxygen tension (PaO2) to 75 mmHg had no effect on cerebral blood flow. However, decreasing the PaO2 below 75 mmHg significantly increased blood flow to the fast-clearing compartment. The greatest increase in blood flow was seen when the arterial PO2 was below 50 mmHg. At an arterial PO2 of 30 mmHg, the cerebral blood flow to the fast-clearing compartment was increased more than 600% above the normoxic level. The magnitude of this increase is much greater in the duck than has been reported for mammals at roughly equivalent arterial oxygen tensions. The ability of avian cerebral blood flow to increase at moderate levels of hypoxia, plus the magnitude of the increase, may play a role in the exceptional tolerance of birds to hypoxia.