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Chapter 9

Chapter 9: Stress and Adaptation .doc

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Peter Helli

PATHOPHYSIOLOGY STRESS AND ADAPTATIONChapter 9 Stress and AdaptationHomeostasisHomeostasis is the purposeful maintenance of a stable internal environment by coordinated physiologic processes that oppose changeThe systems maintaining homeostasis work by negative feedback mechanismsThey consist of a sensor and integratorcomparator and an effectorThe concepts of stress and adaptation have their origin in the complexity of the human body and the interactions between the body cells and its many organ systems These interactions require that a level of homeostasis or constancy be maintained during the many changes that occur in the internal and external environments In effecting a state of constancy homeostasis requires feedback control systems that regulate cellular function and integrate the function of the different body systemsConstancy of the Internal EnvironmentThe environment in which body cells live is the local fluid environment that surrounds each cell Body fluids surrounding the cells and the various organ systems provide the means for exchange between the external and internal environmentsFrom this internal environment body cells receive their nourishment and into this fluid they release their wastesThe contents of the gastrointestinal tract and lungs do not become part of the internal environment until they have been absorbed into extracellular fluidEven a small change in the pH of the body fluids can disrupt the metabolic processes of the individual cellsHomeostasis involves resistance to both internal and external disturbancesControl SystemsPhysiologic control systems regulate body function and maintain homeostasisA homeostatic control system consists of a collection of interconnected components that function to keep a physical or chemical parameter of the body relatively constantNeuroendocrine control systems influence behaviourFeedback systemsNegative feedback mechanisms when the monitored function decreases below the setpoint of the system the feedback mechanism causes the function to increase when the function is increased above the set point the feedback mechanism causes it to decreasePositive feedback mechanisms positive feedback mechanism interjects instability rather than stability into the system It produces a cycle in which the initiating stimulus produces more of the sameThe stress responseGeneral adaptation syndrome general because the response was in reaction to a stressor adaptive because the response was in reaction to a stressor and syndrome because the physical manifestations were coordinated and dependent on each other
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