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Analysis of the Arterial Baroreflex
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For most purposes, the simple “thermostat analogy” provides a sufficient understanding of how the arterial baroreflex operates. However, in certain situations—especially when there are multiple disturbances in the cardiovascular system—a more detailed understanding is helpful. Consequently, the operation of the arterial baroreflex is presented in this appendix with a more formal control system approach.
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The complete arterial baroreceptor reflex pathway is a control system made up of two distinct portions, as shown in Figure E–1: (1) an effector portion, including the heart and peripheral blood vessels; and (2) a neural portion, including the arterial baroreceptors, their afferent nerve fibers, the medullary cardiovascular centers, and the efferent sympathetic and parasympathetic fibers. Mean arterial pressure is the output of the effector portion and simultaneously the input to the neural portion. Similarly, the activity of the sympathetic (and parasympathetic) cardiovascular nerves is the output of the neural portion of the arterial baroreceptor control system and, at the same time, the input to the effector portion. For convenience, we omit continual reference to parasympathetic nerve activity in the following discussion. Throughout, however, an indicated change in sympathetic nerve activity should usually be taken to imply a reciprocal change in the activity of the cardiac parasympathetic nerves.
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A host of reasons why mean arterial pressure increases when the heart and peripheral vessels receive increased sympathetic nerve activity were discussed in this textbook. All this information is summarized by the curve shown in the lower graph in Figure E–1, which describes the operation of the effector portion of the arterial baroreceptor system alone. In Chapter 9, how increased mean arterial pressure acts through the arterial baroreceptors and medullary cardiovascular centers to decrease the sympathetic activity has also been discussed. This information is summarized by the curve shown in the upper graph in Figure E–1, which describes the operation of the neural portion of the arterial baroreceptor system alone.
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When the arterial baroreceptor system is intact and operating as a closed loop, the effector portion and neural portion retain their individual rules of operation, as described by their individual function curves in Figure E–1. Yet in the closed loop, the two portions of the system must interact until they come into balance with each other at some operating point with a mutually compatible combination of mean arterial pressure and sympathetic activity. The analysis of the complete system begins by plotting the operating curves for the neural and effector portions of the systems together on the same graph, as shown in Figure E–2A. To accomplish this superimposition, the graph for the neural portion (the upper graph in Figure E–1) was flipped to interchange its vertical and horizontal axes. Consequently, the neural ...