18.5 , which shows that over an intermediate range of carotid sinus pressures (100 to 180 mm Hg), reciprocal changes are evoked in efferent vagal and sympathetic neural activity. Below this range of carotid sinus pressure, sympathetic activity is intense, and vagal activity is virtually absent. Conversely, above the intermediate range of carotid sinus pressure, vagal activity is intense and sympathetic activity is minimal.
Bainbridge Reflex, Atrial Receptors, and Atrial Natriuretic Peptide
In 1915, Francis A. Bainbridge reported that infusing blood or saline into dogs accelerated their heart rate. This increase did not seem to be tied to arterial blood pressure because the heart rate rose regardless of whether arterial blood pressure did or did not change. However, Bainbridge also noted that the heart rate increased whenever central venous pressure rose sufficiently to distend the right side of the heart. This response is termed the Bainbridge reflex. Bilateral transection of the vagus nerves abolished this response.
Many investigators have confirmed Bainbridge’s observations and have noted that the magnitude and direction of the response depend on the prevailing heart rate. When the heart rate is slow, intravenous infusions of blood or electrolyte solutions usually accelerate the heart. At more rapid heart rates, however, such infusions ordinarily slow the heart. What accounts for these different responses? Increases in blood volume not only evoke Bainbridge reflex but also activate other reflexes (of note, the baroreceptor reflex). These other reflexes tend to elicit opposite changes in heart rate. Therefore, changes in heart rate evoked by an alteration in blood volume are the result of these antagonistic reflex effects (
Fig. 18.6 ). Evidently, the Bainbridge reflex predominates over the baroreceptor reflex when blood volume rises, but the baroreceptor reflex prevails over the Bainbridge reflex when blood volume diminishes.
•Fig. 18.4 Experimentaldatashowingthatheartratedecreasesasarterialpressureisincreased.(AdaptedfromCornishKG,etal.Am J Physiol. 1989;257:r595.)250 200 150 100 50 50 75 100 125 150 Arterial pressure (mm Hg) Heart rate (beats/min)
Both atria have receptors that are affected by changes in blood volume and that influence the heart rate. These receptors are located principally in the venoatrial junctions: in the right atrium at its junctions with the venae cavae and in the left atrium at its junctions with the pulmonary veins. Distention of these atrial receptors sends afferent impulses to the brainstem in the vagus nerves. The efferent impulses are carried from the brainstem to the SA node by fibers from both autonomic divisions.
The cardiac response to these changes in autonomic neural activity is highly selective. Even when the reflex increase in heart rate is large, changes in ventricular contractility are generally negligible. Furthermore, the neurally induced increase in heart rate is not usually accompanied by an increase in sympathetic activity in the peripheral arterioles.