described this reflex in 1918 when he was experimenting on dogs. Bainbridge found that infusing blood or saline into the animal increased heart rate. This phenomenon occurred even if arterial blood pressure did not increase. He further observed that heart rate increased when venous pressure rose high enough to distend the right atrium, but denervation of the vagi to the heart eliminated these effects.
Mechanism of Action
Increased blood volume results in increased venous return to the heart, which leads to increased firing of B-fibers. B-fibers send signals to the brain, which then modulates both sympathetic and parasympathetic pathways to the SA node of the heart, causing an increase in heart rate. "Effects on cardiac contractility and stroke volume are insignificant." Bainbridge reflex can be blocked by atropine and can be abolished by cutting the vagus nerve.
Control of heart rate
The Bainbridge reflex and the baroreceptor reflex control heart rate. The baroreceptor reflex can correct for a change in arterial pressure by increasing or decreasing heart rate. In contrast, the Bainbridge reflex responds to changes in blood volume. The Bainbridge reflex is seen in dogs, but experiment has shown that it is not as significant in primates. There is evidence, however, that the Bainbridge reflex does occur in humans, as in after delivery of an infant when a large volume of uteroplacental blood is put back into the mother's circulation, resulting in tachycardia.
Venous return
As venous return increases, the pressure in the superior and inferior vena cava increase. This results in an increase in the pressure of the right atrium, which stimulates the atrial stretch receptors. These receptors in turn signal the medullary control centers to increase the heart rate. Unusually, this tachycardia is mediated by increased sympathetic activity to the sinoatrial node with no fall in parasympathetic activity. Increasing the heart rate serves to decrease the pressure in the superior and inferior venae cavae by drawing more blood out of the right atrium. This results in a decrease in atrial pressure, which serves to bring in more blood from the vena cavae, resulting in a decrease in the venous pressure of the great veins. This continues until right atrial blood pressure returns to normal levels, upon which the heart rate decreases to its original level.
Bainbridge Reflex is involved in Respiratory Sinus Arrhythmia. During inhalation intrathoracic pressure decreases. It triggers increased venous return which is registered by stretch receptors, which via Bainbridge Reflex increases the heart rate momentarily during inspiration. This is not to be confused with stage 4 of the Valsalva maneuver, in which the release of high intrathoracic pressure previously generated by forced expiration against a closed glottis, now restores venous return and cardiac output into a vasoconstricted circulation, stimulating the vagus nerve and leading to a slowing of the heart, or bradycardia.
