A healthy volunteer breathes a gas mixture with low oxygen content for two minutes. Arterial PO2 falls to 50 mmHg. Peripheral chemoreceptors are strongly stimulated. Which cardiovascular change is expected if the accompanying hyperventilation is allowed to occur normally?

• A Marked bradycardia due to direct vagal stimulation from the carotid bodies
• B No heart rate change because the chemoreceptor reflex affects only ventilation
• C Tachycardia because lung inflation signals from pulmonary stretch receptors override the direct vagal effect of chemoreceptor stimulation ✓
• D Bradycardia caused by activation of the Bainbridge reflex

Explanation

Peripheral chemoreceptors in the carotid and aortic bodies directly excite the vasomotor centre and send vagal impulses that would slow the heart. However, simultaneous hyperventilation stimulates pulmonary stretch receptors whose input inhibits the vagal component, so the net result is tachycardia with vasoconstriction. Bradycardia appears only if ventilation is mechanically fixed, as in the experiment where chemoreceptor stimulation occurs without lung inflation. The Bainbridge reflex responds to atrial stretch from volume loading, not hypoxia.

Reference: Guyton and Hall Textbook of Medical Physiology, 14th ed.

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