Bainbridge Reflex

A scientist named Francis Arthur Bainbridge reported this reflex in 1915 while 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 vagus nerve to the heart eliminated these effects.

The Bainbridge reflex, also called the atrial reflex, increases heart rate due to increased central venous pressure. Stretch receptors in the venoatrial junctions of both atria detect increased blood volume.

Control of Heart Rate

The Bainbridge reflex and the baroreceptor reflex act antagonistically to control heart rate. The baroreceptor reflex acts to decrease heart rate when BP rises. When blood volume is increased, the Bainbridge reflex is dominant; when blood volume is reduced, the baroreceptor reflex is dominant. The Bainbridge reflex is seen in dogs, but experiments have 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 (up to 800 mL) of uteroplacental blood is put back into the mother's circulation, resulting in tachycardia.

Venous Return

As venous return increases, the superior and inferior vena cava pressure increases. This increases the pressure of the right atrium, which stimulates the atrial stretch receptors. These receptors, in turn, signal the medullary control centers to decrease parasympathetic tone via the vagus nerve to the heart, leading to increased heart rate, also known as tachycardia.

Increasing the heart rate decreases the pressure in the superior and inferior vena cava by drawing more blood out of the right atrium. This decreases atrial pressure, which brings in more blood from the venae cavae (the superior and inferior vena cava are collectively called the venae cavae), decreasing the venous pressure of the great veins. This process continues until the right atrial blood pressure returns to normal and the heart rate decreases to its original level.

Atrial Stretch Receptors

In the right atrium, the stretch receptors occur at the junction of the venae cavae. In the left atrium, the junction is at the pulmonary veins. Increasing the stretch of the receptors stimulates both an increase in heart rate and a decrease in vasopressin (a.k.a. anti-diuretic hormone - ADH) secretion from the posterior pituitary. This decrease in vasopressin secretion increases the volume of urine excreted, serving to lower BP. In addition, stretching atrial receptors increases atrial natriuretic peptide (ANP) secretion, which promotes increased water and sodium excretion through the urine.

Respiratory Sinus Arrhythmia

Bainbridge Reflex is involved in Respiratory Sinus Arrhythmia. During inhalation, intrathoracic pressure decreases. It triggers increased venous return, registered by stretch receptors, which, via Bainbridge Reflex, increases the heart rate momentarily during inspiration. This is not to be confused with a Valsalva maneuver, in which high intrathoracic pressure generated by a deep breath and bearing down stimulates the vagus nerve, slowing the heart rate or bradycardia.

Reference textbooks excepts

Miller 9th ed.p. 400-401

The Bainbridge reflex elicits stretch receptors in the right atrial wall and the cavoatrial junction. An increase in right-sided filling pressure sends vagal afferent signals to the cardiovascular center in the medulla. These afferent signals inhibit parasympathetic activity, thereby increasing heart rate. Acceleration of the heart rate also directly affects the SA node by stretching the atrium. The heart rate changes depend on the underlying heart rate before stimulation.

Nagelhout (Nurse Anesthesia) 7th ed.p.491

The Bainbridge reflex is elicited due to increased blood volume in the heart, which causes sympathetic nervous system stimulation. Stretch receptors are located in the right atrium, junction of the vena cava, and pulmonary veins. The SA (sinoatrial) node is involved in this process and can increase heart rate by 10% to 15%. This reflex helps prevent blood sequestration in veins, atria, and pulmonary circulation. Antidiuretic hormone secretion from the posterior pituitary gland is decreased, resulting in reduced circulating volume. The atrial natriuretic peptide is increased, which also promotes dieresis.

Guyton and Hall 14th ed.p. 225, 245

Bainbridge reflex tends to accelerate the heart rate, and when blood volume is increased above normal, the Bainbridge reflex often increases heart rate despite the inhibitory actions of the baroreflexes. A small part of the increased heart rate associated with increased blood volume and atrial stretch is caused by a direct effect of the increased atrial volume to stretch the sinus node; such direct stretch can increase the heart rate as much as 15%. An additional 40% to 60% increase in heart rate is caused by the Bainbridge reflex. The stretch receptors of the atria that elicit the Bainbridge reflex transmit their afferent signals through the vagus nerves to the medulla of the brain. Then efferent signals are transmitted back through vagal and sympathetic nerves to increase the heart rate and strength of heart contraction. Thus, this reflex helps prevent damming of blood in the veins, atria, and pulmonary circulation. 

Costanzo 7th ed.p.170

According to Costanzo 3rd ed.p. 163, information from the low-pressure atrial receptors travels in the vagus nerve to the nucleus solitaries (as does information from the high-pressure arterial receptors involved in the baroreceptor reflex). The difference lies in the response of the medullary cardiovascular centers to the low- and high-pressure receptors. Whereas an increase in pressure at the arterial high-pressure receptors produces a decrease in heart rate (trying to lower the arterial pressure back to normal), an increase in pressure at the venous low-pressure receptors produces an increase in heart rate (Bainbridge reflex). The lo-pressure atrial receptors, sensing that blood volume is too high, direct an increase in heart rate and, thus, an increase in cardiac output; the increase in cardiac output leads to increased renal perfusion and sodium and water excretion.

Conclusion

The consensus from the textbooks is that the Bainbridge reflex is when you see an increased heart rate from increased volume or pressure on the atrial/stretch mechanoreceptors.