Only $35.99/year

Autonomic Control of Blood Pressure

Terms in this set (15)

1. High-pressure baroreceptors: The sensor component of the negative feedback loop consists of a set of mechanoreceptors located at strategic high-pressure sites within the cardiovascular system.

2. The two most important high-pressure loci are the carotid sinus and the aortic arch. Stretching of the distensible vessel walls at either site leads to reflex vasodilation and bradycardia.

3. Peripheral chemoreceptors: located in the carotid and aortic bodies, are in close contact with arterial blood. When arterial pressure falls below a critical level, the receptors become stimulated because diminished blood flow causes decreased oxygen, and excess buildup of carbon dioxide and hydrogen ions that are not removed by the slowly flowing blood. Signals transmitted from the chemoreceptors, along with the baroreceptor fibers, pass through Hering's nerves and the vagus nerves into the vasomotor center to elevate the arterial pressure back toward normal. However, the chemoreceptor reflex is not a powerful arterial pressure controller until the arterial pressure falls below 80 mm Hg (e.g., hemorrhage). Thus, it is at the lower pressures that this reflex becomes important to help prevent further decreases in arterial pressure.

4. Central chemoreceptors in the medulla are sensitive to decreases in brain pH (reflecting an increase in arterial PCO2) and cause an increase in SNS output.

In summary, a low PO2 acting on the peripheral chemoreceptor and a high PCO2 acting on the central chemoreceptor act in concert to enhance vasoconstriction.

Note that the cardiovascular system also has low-pressure baroreceptors that detect changes in venous pressure/volume.
Figure: Activation of the baroreceptors at different levels of arterial pressure.

1. Note that the carotid sinus baroreceptors (which transmit impulses in Hering's nerves) are not stimulated by pressures between 0 and 50 to 60 mm Hg. Above these levels, they respond progressively more rapidly and reach a maximum at about 180 mm Hg.

2. The responses of the aortic baroreceptors are similar to those of the carotid receptors except that they operate, in general, at arterial pressure levels about 30 mm Hg higher (higher threshold means less sensitive)

3. In the normal operating range of arterial pressure (~100 mm Hg) even a slight change in pressure causes a strong change in the baroreflex signal to readjust arterial pressure back toward normal. Thus, the baroreceptor feedback mechanism functions most effectively in the pressure range where it is most needed.

4. The baroreceptors respond rapidly to changes in arterial pressure; in fact, the rate of impulse firing increases in the fraction of a second during each systole and decreases again during diastole. Also, the baroreceptors respond much more to a rapidly changing pressure than to a stationary pressure e.g., if the mean arterial pressure is 150 mm Hg but at that moment is rising rapidly, the rate of impulse transmission may be twice that when the pressure is stationary at 150 mm Hg.

5. Note that the baroreceptor reflex ADAPTS to long-term changes in mean arterial pressure. For example, in hypertension, the set point is increased so the curve is parallel and shifted to the right.