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Arterial baroreceptor control of the circulation during forced dives in ducks (Anas Platyrhynchos var.) Smith, Frank Melvin


When dabbling ducks are involuntarily submerged, arterial vasoconstriction produces a large increase in the peripheral resistance to blood flow which is balanced by a decrease in output of the heart, and arterial blood pressure is maintained. Arterial baroreceptors sense systemic blood pressure, and provide the afferent information to the baroreflex for pressure regulation. The effector limbs of the baroreflex are the same as those involved in the diving responses, and the baroreceptors are likely to be important in the integration of the cardiovascular responses to diving. The purpose of this study was to investigate the role of the arterial baroreceptors in maintaining blood pressure during diving, and in the initiation and maintenance of the diving responses. Baroreceptor function was studied by diving ducks at various times after barodenervation, and by electrically stimulating the central end of one baroreceptor nerve in baroreceptor-denervated animals to simulate a controlled baroreceptor input before and during submersion. Intact baroreceptor innervation was not necessary for the development of peripheral vasoconstriction, but loss of the baroreceptors modified the cardiac response to submersion by impairing the vagally mediated bradycardia. There was no effect of baroreceptor nerve stimulation on peripheral resistance during diving, and the baroreflex operated via the heart rate in modulating blood pressure early in the dive. Later in the dive, stimulation was ineffective in altering either heart rate or blood pressue. Strong chemoreceptor drive results from decreased blood oxygen and increased carbon dioxide levels in the dive, and the results of experiments to determine the mechanism of baroreflex attenuation showed that an interaction between chemoreceptor and baroreceptor inputs may be at least partly responsible for reducing baroreflex effectiveness. The main conclusion from this work is that the arterial baroreceptors contribute to the diving responses through modulation of heart rate, to help balance the fall in cardiac output against the baroreceptor-independent peripheral vasoconstriction in the first minute of forced dives.

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