a. A decrease in arterial pressure decreases stretch on the walls of the carotid sinus.
- Because the baroreceptors are most sensitive to changes in arterial pressure, rapidly decreasing arterial pressure produces more response than low, but unchanging, pressure.
- Additional baroreceptors in the aortic arch respond to increases, but not to decreases, in arterial pressure.
b. Decreased stretch decreases the firing rate of the carotid sinus nerve [Hering's nerve, cranial nerve (CN) IX], which carries information to the vasomotor center in the brain stem.
c. The set point for mean arterial pressure in the vasomotor center is about 100 mm Hg. Therefore, if mean arterial pressure is less than 100 mm Hg, a series of autonomic responses is coordinated by the vasomotor center. These changes will attempt to increase blood pressure toward normal.
d. The responses of the vasomotor center to a decrease in mean arterial blood pressure are coordinated to increase the arterial pressure to 100 mm Hg. The responses are decreased parasympathetic (vagal) outflow to the heart and increased sympathetic outflow to the heart and blood vessels.
- The following four effects attempt to increase the arterial pressure to normal:
(1) T heart rate, resulting from decreased parasympathetic tone and increased sympathetic tone to the SA node of the heart.
(2) T contractility, resulting from increased sympathetic tone to the heart. Together with the increase in heart rate, the increase in contractility produces an increase in cardiac output, which increases arterial pressure.
(3) T vasoconstriction of arterioles, resulting from the increased sympathetic outflow. As a result, TPR will increase, increasing arterial pressure.
(4) T vasoconstriction of veins (venoconstriction), resulting from the increased sympathetic outflow. As the veins constrict, unstressed volume will decrease and, as a result, mean systemic pressure will increase. Increases in mean systemic pressure shift the vascular function curve to the right, producing an increase in cardiac output (see Figure 3-13) and arterial pressure.
e. The baroreceptor mechanism is a negative feedback system. As these mechanisms work together to return mean arterial pressure to normal, there will be increased stretch on the carotid sinus barorecep-tors and decreased signal to the vasomotor center.
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