 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.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 (the afferent pathway of the neural portion of the Bainbridge reflex), which then modulates both sympathetic and parasympathetic pathways to the SA node of the heart (the efferent pathway of the neural portion of the Bainbridge reflex), causing an increase in heart rate.