Arterial Pressure Pulse
100 200 Mean Arterial Pressure (mmHg)
FIGURE 6-6 Effects of arterial pressure on integrated carotid sinus firing rate. Left panel: The threshold for receptor activation occurs at mean arterial pressures of about 60 mm Hg; maximal firing occurs at about 180 mm Hg. Maximal receptor sensitivity occurs at normal mean arterial pressures. The receptor firing-response curve shifts to the right with decreased pulse pressures; therefore, a decrease in pulse pressure at a given mean pressure decreases firing. Right panel: Single receptor firing in response to pulsatile pressure. Receptors fire more rapidly when arterial pressure is rapidly increasing during cardiac systole.
right, thereby decreasing the firing at any given mean arterial pressure.
Maximal carotid sinus sensitivity (the point of greatest slope of the response curve in Figure 6-6) occurs near the "set point" of normal mean arterial pressures (approximately 95 mm Hg in adults). Therefore, small deviations from this set point elicit large changes in baroreceptor firing frequency. This set point, and the entire receptor response curve, is not fixed. Chronic shifts in this curve can occur during hypertension, heart failure, and other disease states. In hypertension, for example, the curve shifts to the right, thereby reducing the firing rate at any given mean arterial pressure. This resetting of the baroreceptor response can occur at the level of the receptors themselves as well as in the brainstem. In arteriosclerosis, the carotid arteries at the region of the carotid sinus become less compliant, and therefore they stretch less in response to changes in arterial blood pressure—this decreases their sensitivity. During exercise, medullary and hypothalamic control centers can modulate autonomic efferent responses at a given level of baroreceptor firing, thereby resetting arterial pressure to a higher level.
Receptors located within the aortic arch function similarly to carotid sinus receptors; however, they have a higher threshold pressure for firing and are less sensitive than the carotid sinus receptors. Therefore, the aortic arch baroreceptors serve as secondary barore-ceptors, with the carotid sinus receptors normally being the dominant arterial barorecep-tor.
To understand how the baroreceptor reflex operates, consider the events that occur in response to a decrease in arterial pressure (mean, pulse, or both) when a person suddenly stands up (Figure 6-7). When upright posture is suddenly assumed from the supine position, gravity causes venous blood pooling below the heart, particularly in the legs (see Chapter 5). This decreases venous return, central venous pressure, and ventricular preload, leading to a fall in cardiac output and arterial blood pressure. Decreased stretching of baroreceptors results in decreased baroreceptor firing. The "cardiovascular center" within
Decreased Arterial Pressure
_+ t Sympathetic
Decreased Receptor Firing
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