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The homeostatic control system maintains a relatively constant body temperature when room temperature decreases. This flow diagram is typical of those used throughout the remainder of this book to illustrate homeostatic systems, and several conventions should be noted. (See also the legend for Figure 7-4.) The "begin" sign indicates where to start. The arrows next to each term within the boxes denote increases or decreases. The arrows connecting any two boxes in the figure denote cause and effect; that is, an arrow can be read as "causes" or "leads to." (For example, decreased room temperature "leads to" increased heat loss from the body.) In general, one should add the words "tends to" in thinking about these cause-and-effect relationships. For example, decreased room temperature tends to cause an increase in heat loss from the body, and curling up tends to cause a decrease in heat loss from the body. Qualifying the relationship in this way is necessary because variables like heat production and heat loss are under the influence of many factors, some of which oppose each other.

for the skeletal muscular contractions that constitute shivering produce large quantities of heat.

Indeed, heat production may transiently exceed heat loss so that body temperature begins to go back toward the value existing before the room temperature was lowered (Figure 7-2). It eventually stabilizes at a temperature a bit below this original value; at this new steady state, heat input and heat output are both higher than their original values but are once again equal to each other.

The thermoregulatory system just described is an example of a negative-feedback system, in which an increase or decrease in the variable being regulated brings about responses that tend to move the variable in the direction opposite ("negative" to) the direction of the original change. Thus, in our example, the decrease in body temperature led to responses that tended to increase the body temperature—that is, move it toward its original value.

Negative-feedback control systems are the most common homeostatic mechanisms in the body, but there is another type of feedback known as positive feedback in which an initial disturbance in a system sets off a train of events that increase the disturbance even further. Thus positive feedback does not favor stability and often abruptly displaces a system away from its normal set point. As we shall see, several

-Set point

Error signal

-Set point

Error signal

Set Point Reset Fever

Time

FIGURE 7-2

Changes in internal body temperature during exposure to a low external environmental temperature. As long as the environmental perturbation persists, the homeostatic responses do not return the regulated variable completely to its original value. The deviation from the original value is called the error signal.

Time

FIGURE 7-2

Changes in internal body temperature during exposure to a low external environmental temperature. As long as the environmental perturbation persists, the homeostatic responses do not return the regulated variable completely to its original value. The deviation from the original value is called the error signal.

PART TWO Biological Control Systems

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

PART TWO Biological Control Systems important positive-feedback relationships occur in the body, contractions of the uterus during labor being one example.

Note that in our thermoregulatory example the negative-feedback system did not bring the person's temperature back completely to its original value. This illustrates another important generalization about homeostasis: Homeostatic control systems do not maintain complete constancy of the internal environment in the face of continued change in the external environment, but can only minimize changes. This is the reason we have said that homeostatic systems maintain the internal environment relatively stable. The explanation is that as long as the initiating event (exposure to cold, in our example) continues, some change in the regulated variable (the decrease in body temperature, in our example) must persist to serve as a signal to maintain the homeostatic responses. (This last statement will be qualified below.) Such a persisting signal is termed an error signal (Figure 7-2). This situation applies only when the initiating event continues; thus, in our example if the external temperature eventually goes back up to its original value, the homeostatic systems will be able to restore body temperature completely back to its original value.

It is essential to recognize that normally a control system does not overcompensate (that is, drive the system beyond the normal set point to create another physiological imbalance).

Inherent in the concept of error signals is another generalization about homeostasis: Even in reference to one individual, thus ignoring variation among persons, any regulated variable in the body cannot be assigned a single "normal" value but has a more-or-less narrow range of normal values, depending on the magnitude of the changes in the external conditions and the sensitivity of the responding homeostatic system. The more precise the mechanisms for regulating a variable are—that is, the smaller the error signal need be to drive the system—the narrower is the range. For example, the temperature-regulating systems of the body are extremely sensitive so that body temperature normally varies by only about 1°C even in the face of marked changes in the external environment or heat production during exercise.

As we have seen, perturbations in the external environment can displace a variable from its preexisting set point. In addition, the set points for many regulated variables can be physiologically altered or reset; that is, the values that the homeostatic control systems are "trying" to keep relatively constant can be altered. A common example is fever, the increase in body temperature that occurs in response to infection and that is analogous to raising the setting of your house's thermostat. The homeostatic control systems regulating body temperature are still functioning during a fever, but they maintain the temperature at a higher value. We shall see in Chapter 20 that this regulated rise in body temperature is adaptive for fighting the infection.

The fact that set points can be reset adaptively, as in the case of fever, raises important challenges for medicine, as another example illustrates. Plasma iron concentration decreases significantly during many infections. Until recently it was assumed that this decrease is a symptom caused by the infectious organism and that it should be treated with iron supplements. In fact, just the opposite is true: As described in Chapter 20, the decrease in iron is brought about by the body's defense mechanisms and serves to deprive the infectious organisms of the iron they require to replicate. Several controlled studies have shown that iron replacement can make the illness much worse. Clearly it is crucial to distinguish between those deviations of homeostatically controlled variables that are truly part of a disease and those that, through resetting, are part of the body's defenses against the disease.

The examples of fever and plasma iron concentration may have left the impression that set points are reset only in response to external stimuli, such as the presence of bacteria, but this is not the case. Indeed, as described in the next section, the set points for many regulated variables change on a rhythmical basis every day; for example, the set point for body temperature is higher during the day than at night.

Although the resetting of a set point is adaptive in some cases, in others it simply reflects the clashing demands of different regulatory systems. This brings us to one more generalization: It is not possible for everything to be maintained relatively constant by homeo-static control systems. In our example, body temperature was kept relatively constant, but only because large changes in skin blood flow and skeletal-muscle contraction were brought about by the homeostatic control system. Moreover, because so many properties of the internal environment are closely interrelated, it is often possible to keep one property relatively constant only by moving others farther from their usual set point. This is what we meant by "clashing demands."

The generalizations we have given concerning homeostatic control systems are summarized in Table 7-1. One additional point is that, as is illustrated by the regulation of body temperature, multiple systems frequently control a single parameter. The adaptive value of such redundancy is that it provides much greater fine-tuning and also permits regulation to occur even when one of the systems is not functioning properly because of disease.

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

Homeostatic Mechanisms and Cellular Communication CHAPTER SEVEN

TABLE 7-1 Some Important Generalizations

About Homeostatic Control Systems

1. Stability of an internal environmental variable is achieved by balancing inputs and outputs. It is not the absolute magnitudes of the inputs and outputs that matter but the balance between them.

2. In negative-feedback systems, a change in the variable being regulated brings about responses that tend to move the variable in the direction opposite the original change—that is, back toward the initial value (set point).

3. Homeostatic control systems cannot maintain complete constancy of any given feature of the internal environment. Therefore, any regulated variable will have a more-or-less narrow range of normal values depending on the external environmental conditions.

4. The set point of some variables regulated by homeostatic control systems can be reset—that is, physiologically raised or lowered.

5. It is not possible for everything to be maintained relatively constant by homeostatic control systems. There is a hierarchy of importance, such that the constancy of certain variables may be altered markedly to maintain others at relatively constant levels.

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Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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Responses

  • ren
    How does homeostasis control body temperature flow diagram?
    8 years ago
  • Samantha
    How homeostatic responses in the internal environment may have changed during exercise?
    8 years ago

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