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FIGURE 2 Functional organization of the carotid sinus reflex. CSN, carotid sinus nerve.

to the arterioles and veins so that peripheral resistance falls and the unstressed volume increases. All of these changes will act to reduce arterial pressure. Cutting the sinus nerves would be interpreted as a precipitous drop in MAP by the vasomotor center, hence, the pressor response. Similarly, repetitive stimulation of the nerve would be interpreted as an increased MAP.

A negative feedback relationship exists between carotid sinus pressure and MAP as a result of the baroreceptor reflex. This negative feedback can be appreciated by dissociating carotid sinus pressure from MAP. This can be accomplished easily by independently perfusing the carotid sinus via a cannula, while MAP is monitored. When the carotid sinus is independently perfused, MAP becomes inversely related to pressure in the carotid sinus, as shown in Fig. 3.

Additional baroreceptors, the aortic baroreceptors, are located in the wall of the aortic arch. Pressure

Isolated Carotid Sinus Pressure —►

FIGURE 3 Functional relationship between carotid sinus pressure and mean arterial pressure in an isolated carotid sinus preparation.

Isolated Carotid Sinus Pressure —►

FIGURE 3 Functional relationship between carotid sinus pressure and mean arterial pressure in an isolated carotid sinus preparation.

information from these receptors is carried to the sensory region of the vasomotor center via vagal afferent fibers. Both the carotid and aortic baro-receptors monitor arterial pressure, and the information from each is combined in the vasomotor center. The dynamic ranges of these two receptor sets are different because of the structure of the receptors themselves. As shown in Fig. 4, the carotid baror-eceptors provide the vasomotor center with information regarding arterial pressure within the range of 50-200 mm Hg, whereas the aortic baroreceptors can only provide pressure information within the range of 100-200 mm Hg.

Both the carotid and aortic baroreceptors are rate sensitive, which means that a rising pressure causes a more rapid rate of firing than a steady pressure. As a result, the carotid and aortic baroreceptors provide the vasomotor center with information regarding both MAP and arterial pulse pressure. Recordings from single baroreceptor afferents demonstrate that information pertaining to both of these pressures is encoded within the pattern of action potentials generated by the baroreceptors. It has been further demonstrated that the vasomotor center uses both mean and pulse pressure information to modulate cardiovascular effectors. Figure 5 illustrates this principle. As in Fig. 3, the carotid sinus is perfused by a pump and MAP is measured. The family of curves represents different pulse pressures in the carotid sinus. Note that the more pulsatile the pressure in the carotid sinus, the greater the inhibitory effect seen on arterial pressure. Thus, the functional relationship between carotid sinus pressure and MAP is shifted when carotid sinus pressure is pulsatile, and that effect is most pronounced at lower pressures.

FIGURE 4 Baroreceptor dynamic range. Pressure ranges over which the carotid and aortic baroreceptors can monitor arterial blood pressure.

50 100 150 200

Baroreceptor Pressure (mm Hg)

FIGURE 4 Baroreceptor dynamic range. Pressure ranges over which the carotid and aortic baroreceptors can monitor arterial blood pressure.

Importance of the Baroreceptor Reflex

The carotid sinus nerves and the vagal nerves from the aortic arch are often called buffer nerves because, by monitoring arterial pressure, they help the vasomotor center to buffer any change in MAP associated with daily activity. This buffer function can be readily demonstrated with the frequency distribution of arterial pressure over a 24-hr period for a normal dog and the pressure distribution recorded in the same dog several weeks after baroreceptor denervation (Fig. 6). Figure 6 reveals that when the buffer nerves are intact, MAP rarely deviates more than 10 or 15 mm Hg away from the 100-mm Hg set point. However, when the buffer nerves are sectioned, pressure varies widely throughout the day, often 50 mm Hg or more above or below the 100-mm Hg set point. Note from the figure that denervation of the baroreceptors does not raise the average blood pressure on a long-term basis. The reason is that the baroreceptor reflex adapts to the pressure

FIGURE 5 Pulse pressure modulation of the functional relationship between mean arterial pressure and systemic vascular resistance resulting from the baroreceptor reflexes.

input after a few days. This experiment vividly demonstrates that long-term control of blood pressure is regulated by other mechanisms, as explained later.

It is not a coincidence that the baroreceptors are located in the upper thorax and neck. It is vital that the brain and heart have a continuous blood flow because of their high metabolic rates. Interruption of that flow for even a brief period can spell disaster for an individual. To protect those two circulations, the baroreceptors are located close to the brain (carotid sinuses) and the heart (aortic baroreceptors). In an erect adult, there may be a 100-mm Hg difference between the pressure in the cerebral arteries and those in the foot because of the hydrostatic column between them. The strategic location of the baroreceptors prevents pressure differences within the vascular system from depriving the brain and heart of an adequate perfusion pressure.

As we will see in Chapter 17, the blood flow of many organs is closely coupled to their metabolic needs via local control systems. Organs exhibiting such a tight local control include the brain, the heart, the digesting intestines, and exercising skeletal muscles. Other organs such as the skin, resting skeletal muscle, or nondigesting intestines tend to be well perfused with respect to their nutritional needs, and their blood flows are greatly influenced by the autonomic nerves. The latter vascular beds participate in baroreceptor reflexes from the vasomotor center by sacrificing their flow when MAP falls. When MAP begins to fall, vascular resistances in the gut and skeletal muscle increase to buffer the changes in MAP, while local control mechanisms in the heart and brain override any central influences on their blood flows. Skeletal muscle and gut are more readily recruited into the baroreceptor reflex when they are resting, with most neural commands being overridden when they are active and a high blood flow is required.

The cardiovascular control system is also able to handle perturbations associated with stressful situations such as an extreme drop in cardiac output resulting from hemorrhage. Under such a stress, peripheral sympathetic stimulation will be so intense that even the vascular beds under strong local control (except the brain and heart) will constrict. The resulting increase in total peripheral resistance will be at the expense of tissues that can tolerate a period of reduced blood flow and divert vital flow to the brain and heart. If that condition persists, however, the organs that sacrifice their flows will experience ischemic tissue injury.

Low-Pressure Receptors

The atria and pulmonary arteries also contain stretch receptors that are often referred to as low-pressure

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