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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:

Table 1-1Major Components and Functions of the Body Systems

  • 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.

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

Negative Feedback Control

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):

Table 1-2Some Examples of Physiologic Controlled Variables

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