The relative roles of the aortic and carotid sinus nerves in the rabbit in the control of respiration and circulation during arterial hypoxia and hypercapnia.

1. The respiratory and circulatory effects of graded arterial hypoxia, alone or with superadded hypercapnia, were studied in four groups of unanaesthetized rabbits including normal animals and those with selective section of the aortic nerves, selective section of the carotid sinus nerves and section of both sets of nerves.2. When measured 2-4 days after selective section of the carotid sinus nerves the resting respiratory minute volume and arterial P(O2) were lower and the P(CO2) higher than normal. These effects were not observed after selective section of the aortic nerves. Selective aortic nerve section, and selective carotid sinus nerve section each produced a similar increase in the resting arterial pressure and heart rate, but were without effect on the resting cardiac output.3. During arterial hypoxia reflex respiratory and circulatory effects ascribable to arterial chemoreceptor stimulation (hyperventilation, bradycardia, vasoconstriction) were mediated for the most part through the carotid sinus nerve. In animals with only the aortic nerves intact the circulatory response was determined largely by the opposing effects of aortic baroreceptor reflexes and the local peripheral dilator action of hypoxia.4. The circulatory effects of hyperventilation induced by hypercapnia during arterial hypoxia, in animals with both aortic and carotid sinus nerves cut were small.5. The results suggest that relatively few chemoreceptor fibres originate from the aortic region in the rabbit, though the carotid sinus and aortic nerves both contain baroreceptor fibres.