The Bainbridge reflex differs from the baroreceptor reflex in its response to blood volume expansion. Which statement best describes the mechanism and direction of the Bainbridge reflex?

• A Atrial stretch receptors (mechanoreceptors at the venoatrial junctions) detect increased right atrial pressure and reflexly increase heart rate via vagal withdrawal and sympathetic activation ✓
• B Ventricular mechanoreceptors detect increased wall tension and reflexly decrease heart rate to prevent overfilling
• C Aortic arch baroreceptors detect increased blood pressure and reflexly decrease heart rate (same as carotid sinus reflex)
• D Atrial stretch activates ANP release, which reflexly increases heart rate via cGMP signalling

Explanation

The Bainbridge reflex (atrial reflex, or cardioacceleratory reflex) is triggered by stretch of mechanoreceptors at the SA node region and the venoatrial junctions, signalling via vagal afferents to the medullary cardiovascular centres. The response is increased heart rate — the opposite of the baroreceptor reflex. When venous return increases (e.g., lying down, fluid infusion), atrial stretch triggers tachycardia to increase cardiac output and 'clear' the elevated preload. This distinguishes it from the baroreceptor reflex (which is pressure-mediated and causes bradycardia). The Bainbridge reflex helps explain postural heart rate changes and contributes to heart rate variability.

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

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