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HOMEOSTASIS

  1. Medical physiology is about how the body systems function and how they are controlled.

  2. Homeostasis is the maintenance of a stable internal environment and requires integration of organ system functions (Table 1-1).

  3. Negative feedback control.

    1. The stability of the body's internal environment is defined by the maintenance of physiologic controlled variables within narrow normal ranges (Table 1-2).

    2. Minimal variation in a controlled variable is explained by the presence of negative feedback control mechanisms.

    3. Negative feedback responses counter deviations of a controlled variable from its normal range; this is the major control process used to maintain homeostasis.

    4. A negative feedback control system has the following elements (Figure 1-1):

      1. A set point value, which is at the center of the normal range and is treated by the control system as the target value.

      2. Sensors that monitor the controlled variable.

      3. A comparator, which interprets input from the sensors to determine when deviations from the set point have occurred. The comparator initiates a counter response.

      4. Effectors are the mechanisms that restore the set point.

    5. Using the control of blood pressure as an example:

      1. The controlled variable is mean arterial blood pressure (MAP).

      2. The normal set point for MAP is approximately 95 mm Hg.

      3. Pressure sensors are located in the carotid sinus and relay information to a comparator located in the central nervous system.

      4. If MAP suddenly changes, the activity of effectors (e.g., cardiac contractility, vascular tone, and urinary fluid excretion) is altered to restore normal blood pressure.

  4. The internal environment.

    1. The purpose of homeostasis is to provide an optimal fluid environment for cellular function.

    2. The body fluids are divided into two major functional compartments:

      1. Intracellular fluid (ICF) is the fluid inside cells.

      2. Extracellular fluid (ECF) is the fluid outside cells, which is subdivided into the interstitial fluid and the blood plasma.

    3. The concept of an internal environment in the body correlates with the interstitial fluid bathing cells.

    4. There is free exchange of water and small solutes in the ECF between interstitial fluid and plasma across the blood capillaries.

    5. Exchange between interstitial fluid and ICF is highly regulated and occurs across cell membranes.

    6. The volume of total body water is approximately 60% of the body weight in men and 50% in women.

      1. About 60% of the total body water is ICF and 40% is ECF (Figure 1-2).

      2. Approximately 80% of the ECF is interstitial fluid and the remaining 20% is blood plasma.

      3. ECF is high in NaCl and low in K+, whereas ICF is high in K+ and low in NaCl.

      4. Interstitial fluid is similar in composition to plasma, except that interstitial fluid has almost no protein.

      5. Osmolarity is the same in all compartments.

    7. imageFluid can move freely from the interstitial to plasma compartments and helps to maintain blood volume during hemorrhage.

      1. Because approximately 80% of the ECF is interstitial fluid and 20% is blood plasma, a hemorrhaging patient must lose about 5 L of ...

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