Effect of progressive hypoxia on the respiratory & cardiovascular system of chickens.

1. During the initial stages of progressive hypoxia the intact, unanaesthetized chicken shows increases in heart rate and respiratory frequency with no change in arterial blood pressure and oxygen consumption. During the later stages, heart rate, diastolic and mean blood pressure and oxygen consumption fall, while respiratory frequency increases further.2. Following bilateral cervical vagotomy and adrenergic beta-receptor blockage there is no tachycardia, but the late bradycardia and fall in blood pressure do occur during progressive hypoxia. Respiratory frequency remains at a low level after vagotomy.3. It is suggested that the initial tachycardia is dependent on both the sympathetic and parasympathetic nervous systems, and that the former helps maintain arterial pressure during the early stages of hypoxia. Bradycardia and hypotension seem to be due to anoxia itself, and the vagus is essential for the increase in respiratory frequency.