19 - Chapter 19. PHYSIOLOGICAL CONTROL SYSTEMS important...

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2007 version – page 143 Chapter 19. PHYSIOLOGICAL CONTROL SYSTEMS The preceding sections of this text have provided an overview of the chemical makeup of the body as a whole, the variety of compartments used for convenience in description, and the specific nature of cells. Although the details provided are sometimes the result of research performed very recently, it has long been recognized that living creatures exhibit some features that are truly distinct from inanimate objects. The background and a more contemporary overview of controls operating in physiologic systems are provided here. After critical mechanisms are developed in the next several chapters, aspects of how animals deal with challenges to their well- being will be described in Chapters 60-63. A. An introduction to homeostasis In 1879, the French physiologist Claude Bernard was the first to formally distinguish between the milieu interieur , or the internal environment of animals, and the milieu exterieur , or the world at large. In Bernard's analysis, importance was placed on the notion that living beings expended effort to preserve the internal environment, quite distinct from that prevailing outside. For 50 years, physiologists explored the mechanisms that served in this protective role and reinterpreted older discoveries, most of which had been made in the preceding 300 years. In 1929, an American physiologist, Walter B. Cannon, assembled this great body of knowledge into the most important conceptual dictum of this discipline. Cannon hypothesized that the central purpose of physiologic mechanisms was to provide constancy of the internal environment. The internal milieu is potentially subjected to external influences that threaten its stability; physiologic mechanisms continually operate to oppose and offset these changes. Cannon's recognition of the relative stability of virtually all physiologic variables such as body temperature in homeotherms, blood glucose concentrations, blood gas levels, and the like, has dominated all subsequent developments in physiology. The terms steady state and homeostasis are widely used to describe this relative constancy, and mechanisms providing for this stability are called homeostatic . The concept of homeostasis is exceedingly valuable in trying to understand the near instantaneous adjustments that occur in almost all physiologic quantities. In simplified terms, any factor that tends to disturb a variable, such as body temperature, away from its steady state value will be opposed by homeostatic mechanisms that attempt to reestablish the normal value. As shown in Figure 19-1, a perturbation, or disturbance, of the steady state triggers compensatory reactions that serve to restore the steady state. For example, an animal suddenly subjected to excessive ambient temperatures may experience an increase in body temperature, but this is promptly offset by the activation of heat losing ( thermolytic ) mechanisms.
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2007 version – page 144 Figure 19-1. Physiological adjustments.
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This note was uploaded on 03/23/2009 for the course ANSCI 1110 taught by Professor Brucecurrie during the Fall '08 term at Cornell University (Engineering School).

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19 - Chapter 19. PHYSIOLOGICAL CONTROL SYSTEMS important...

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