60 - K STRESS AND DEFENSE MECHANISMS Most of the material...

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K STRESS AND DEFENSE MECHANISMS Most of the material presented so far in this book has been concerned with the structural and mechanistic basis for normal animal function. The emphasis has been placed on fundamental chemical and physical principles that permit close monitoring and protection of the stability of the internal environment. Some examples of major breakdowns of the normal mechanisms have been described. In most cases the disturbances represented pathologies that stemmed not from any infectious disease vector but rather from inappropriate operation of an otherwise normal physiologic mechanism. Such pathologies reflect serious departures from homeostatic regulation. . 2007 version – page 462
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Chapter 60. STRESS, ADAPTATION, HOMEORHESIS, AND PATHOGENE S IS The general concepts of homeostasis have been illustrated for a variety of physiologic variables. For example, in Chapters 32 through 37 a variety of mechanisms that regulate blood gas status were integrated as we saw how the cardiovascular, respiratory, and renal systems cooperate with each other to achieve a common objective. The first introduction of homeostasis, in Chapter 19, described the shortcomings of the notion of an absolutely constant steady state. The concept of a physiologic comfort zone was offered as an alternative. Physiologic variables can take any value within the comfort zone without invoking compensatory reactions. When a variable is perturbed to the extent that it moves out of the comfort zone, reactive, compensatory mechanisms come into play, and the variable is brought back toward a steady-state value. These adjustments are quite normal and pose no great threat or challenge to the animal's body. When physiologic variables are greatly disturbed, however, the animal is stressed , and much greater effort is required to offset and correct the change. Animals then invoke more robust, or vigorous compensatory and protective mechanisms to restore stability of their internal environment because, if correction is not attempted or is unsuccessful, a pathophysiologic state can develop. In extreme cases, these deleterious changes in certain critical variables can be life-threatening and may actually cause death. This was illustrated, most dramatically in Box 34- 3, in the case of acute hypovolemic shock. In stress, animals divert their resources so that the corrective mechanisms receive priority. Other activities, such as the productive functions so important in animal agriculture - growth, fiber production, reproduction, egg-laying and lactation - are compromised until the stress is alleviated. For example, a heat-stressed animal will seek shade to minimize solar radiant heat gain. It is also likely to stop eating. This is partly because foraging activity is curtailed in favor of staying in the shade. Additionally, the heat production associated with the activity of foraging, digesting, and absorbing
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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.

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60 - K STRESS AND DEFENSE MECHANISMS Most of the material...

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