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1. In the spontaneously breathing anaesthetized dog, the systemic circulation was perfused at constant blood flow; there was no pulmonary blood flow and the systemic arterial blood PO2 and PCO2 were controlled independently by an extracorporeal isolated pump-perfused donor lung preparation. The carotid and aortic bodies were separately perfused at constant pressure with blood of the same composition as perfused the systemic circulation.
2. Apnoeic asphyxia, produced by stopping the recipient animal's lung movements and, at the same time, making the blood perfusing the systemic circulation and the arterial chemoreceptors hypoxic and hypercapnic by reducing the ventilation of the isolated perfused donor lungs, caused an increase in systemic vascular resistance.
3. While the systemic arterial blood was still hypoxic and hypercapnic, withdrawal of the carotid and aortic body `drive' resulted in a striking reduction in systemic vascular resistance. Re-establishing the chemoreceptor `drive' immediately increased the vascular resistance again.
4. Apnoeic asphyxia carried out while the carotid and aortic bodies were continuously perfused with oxygenated blood of normal PCO2 had little or no effect on systemic vascular resistance.
5. The systemic vasoconstrictor response produced by apnoeic asphyxia was reduced or abolished by re-establishing the recipient animal's lung movements, and this effect occurred in the absence of changes in the composition of the blood perfusing the systemic circulation and arterial chemoreceptors. This abolition of the vasoconstriction was due to a pulmonary reflex.
6. Apnoeic asphyxia slowed the rate of the beating atria due to excitation of the carotid and aortic body chemoreceptors. This response can be over-ridden by an inflation reflex arising from the lungs.
7. It is concluded that the cardiovascular responses observed in apnoeic asphyxia are due, at least in part, to primary reflexes from the carotid and aortic body chemoreceptors engendered by arterial hypoxia and hypercapnia. The appearance of these responses is, however, dependent upon there being no excitation of a pulmonary (inflation) vagal reflex.
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