The atrial reflex adjusts heart rate in response to which parameter?

• A. Venous return
• B. Systemic arterial pressure
• C. Oxygen concentration
• D. Blood pH

Answer: (A)

The heart adjusts its rate in response to how much blood is returning to it, and this question centers on the atrial reflex, also known as the Bainbridge reflex. When venous return increases, the right atrium stretches more. Specialized stretch receptors in the atrial walls detect this stretch and send signals to the brainstem via the vagus nerve. In response, the heart rate rises because the brain increases sympathetic influence on the sinoatrial node and reduces parasympathetic activity. This helps match the heart’s pumping rate to the larger inflow, preventing excessive atrial pooling and maintaining cardiac output.

This reflex specifically concerns venous return. Systemic arterial pressure is mainly handled by baroreceptors, oxygen concentration and blood pH affect heart rate through chemoreceptor pathways or other reflexes, but not the atrial stretch reflex itself.