Aspects of the Relative Roles of Peripheral Vasoconstriction and Vagal Bradycardia in the Establishment of the “Diving Reflex” in Ducks

The effects of α‐adrenergic (Phenoxybenzamine) and parasympathetic (Atropin) blockade on the cardiovascular response to diving in the duck has been studied. With no significant reduction in the immediate, slight fall in heart rate, a‐adrenergic blockade caused peripheral vasodilatation, and virtually abolished the more delayed phase of profound bradycardia during diving. Atropinization eliminated both elenients of the diving bradycardia., while the arterial and venous pressure greatly increased, suggesting that intense activation of the vasoconstrictor fibres was maintained during submersion. It is concluded that the gradual but profound intensification of the initial, slight bradycardia in the dividing duck is a matter of a secondary reflex adjustment to the pressure level.