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Homeostasis

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Medical physiology is concerned with how a state of health and wellness is maintained in a person and, therefore, it takes a global view of how the body systems function and how they are controlled. There are 10 body systems, each with unique contributions to body function (Table 1-1). However, it is the integration of the body systems that allows the creation of a stable internal environment in which cells are able to function. For example, the maintenance of normal blood pressure requires the integration of several organ systems:

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Table 1-1   Major Components and Functions of the Body Systems 
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  • The major determinants of blood pressure in the cardiovascular system are the rate of blood flow (cardiac output) and the vascular resistance.

  • The volume of blood is a key determinant of blood pressure and is controlled by a balance between fluid and salt intake, via the gastrointestinal system, and their excretion via the renal system.

  • Appetite and thirst are controlled by the nervous system, which, together with the endocrine system, integrates the body systems.

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Such ability to maintain a stable internal environment is a central concept in physiology and is referred to as homeostasis.

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Negative Feedback Control

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The stability of the body's internal environment is defined by the maintenance of several physiologic controlled variables within narrow normal ranges (Table 1-2). The characteristic of minimal variation in a controlled variable is explained by the presence of negative feedback control mechanisms. Negative feedback is the initiation of responses that counter deviations of a controlled variable from its normal range and is the major control process used to maintain a stable internal environment. A negative feedback control system contains the following elements (Figure 1-1):

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Table 1-2   Some Examples of Physiologic Controlled Variables 

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