Homeostasis and Negative Feedback

Homeostasis17 (ho-me-oh-STAY-sis) is one of the theories that will arise most frequently in this book as we study mechanisms of health and disease. The human body has a remarkable capacity for self-restoration. Hippocrates commented that it usually returns to a state of equilibrium by itself, and people recover from most illnesses even without the help of a physician. This tendency results from homeostasis, the ability to detect change and activate mechanisms that oppose it.

French physiologist Claude Bernard (1813-78) observed that the internal conditions of the body remain fairly stable even when external conditions vary greatly. For example, whether it is freezing cold or swelteringly hot outdoors, the internal temperature of your body stays within a range of about 36° to 37°C (97°-99°F). American physiologist Walter Cannon (1871-1945) coined the term homeostasis for this tendency to maintain internal stability. Homeostasis has been one of the most enlightening concepts in physiology. Physiology is largely a group of mechanisms for maintaining homeostasis, and the loss of homeostatic control tends to cause illness or death. Patho-physiology is essentially the study of unstable conditions that result when our homeostatic controls go awry.

Do not, however, overestimate the degree of internal stability. Internal conditions are not absolutely constant but fluctuate within a limited range, such as the range of body temperatures noted earlier. The internal state of the body is best described as a dynamic equilibrium (balanced change), in which there is a certain set point or average value for a given variable (such as 37°C for body temperature) and conditions fluctuate slightly around this point.

The fundamental mechanism that keeps a variable close to its set point is negative feedback—a process in which the body senses a change and activates mechanisms that negate or reverse it. By maintaining stability, negative feedback is the key mechanism for maintaining health.

These principles can be understood by comparison to a home heating system (fig. 1.11). Suppose it is a cold winter day and you have set your thermostat for 20°C (68°F)— the set point. If the room becomes too cold, a temperature-sensitive switch in the thermostat turns on the furnace. The temperature rises until it is slightly above the set point, and then the switch breaks the circuit and turns off the furnace. This is a negative feedback process that reverses the falling temperature and restores it to something close to the set point. When the furnace turns off, the temperature slowly drops again until the switch is reactivated—thus, the furnace cycles on and off all day. The room temperature does not stay at exactly 20°C but fluctuates a few degrees either way—the system maintains a state of dynamic equilibrium in which the temperature averages 20°C and deviates from the set point by only a few degrees. Because feed-

Room temperature falls to 66°F (19°C)

Room cools down

Thermostat activates furnace

Thermostat activates furnace

Thermostat shuts off furnace

Thermostat shuts off furnace

Negative Feedback Thermostat Thermostat Homeostasis

Figure 1.11 Negative Feedback in a Home Heating System.

  • a) The negative feedback loop that maintains room temperature.
  • b) Fluctuation of room temperature around the thermostatic set point. What component of the heating system acts as the sensor? What component acts as the effector?

Figure 1.11 Negative Feedback in a Home Heating System.

  • a) The negative feedback loop that maintains room temperature.
  • b) Fluctuation of room temperature around the thermostatic set point. What component of the heating system acts as the sensor? What component acts as the effector?

back mechanisms alter the original changes that triggered them (temperature, for example), they are often called feedback loops.

Body temperature is also regulated by a "thermostat"— a group of nerve cells in the base of the brain that monitors the temperature of the blood. If you become overheated, the thermostat triggers heat-losing mechanisms (fig. 1.12). One of these is vasodilation (VAY-zo-dy-LAY-shun), the widening of blood vessels. When blood vessels of the skin dilate, warm blood flows closer to the body surface and loses heat to the surrounding air. If this is not enough to return your temperature to normal, sweating occurs; the evaporation of

Saladin: Anatomy & I 1. Major Themes of I Text I I © The McGraw-Hill

Physiology: The Unity of Anatomy and Physiology Companies, 2003 Form and Function, Third Edition

18 Part One Organization of the Body o O

  1. 50C (99.50F)
  2. 00C (98.60F)
  3. 50C (97.70F)

18 Part One Organization of the Body

  1. 50C (99.50F)
  2. 00C (98.60F)
  3. 50C (97.70F)

Sweating

Set point

< Vasoconstriction -Shivering

Figure 1.12 Negative Feedback in Human Thermoregulation. Negative feedback keeps the human body temperature homeostatically regulated within about 0.5°C of a 37°C set point. Sweating and cutaneous vasodilation lower the body temperature; shivering and cutaneous vasoconstriction raise it. Why does vasodilation reduce the body temperature?

Sweating

Set point

< Vasoconstriction -Shivering

Figure 1.12 Negative Feedback in Human Thermoregulation. Negative feedback keeps the human body temperature homeostatically regulated within about 0.5°C of a 37°C set point. Sweating and cutaneous vasodilation lower the body temperature; shivering and cutaneous vasoconstriction raise it. Why does vasodilation reduce the body temperature?

water from the skin has a powerful cooling effect (see insight 1.3). Conversely, if it is cold outside and your body temperature drops much below 37°C, these nerve cells activate heat-conserving mechanisms. The first to be activated is vasoconstriction, a narrowing of the blood vessels in the skin, which serves to retain warm blood deeper in your body and reduce heat loss. If this is not enough, the brain activates shivering—muscle tremors that generate heat.

Insight 1.3 Medical History

Men in the Oven

English physician Charles Blagden (1748-1820) staged a rather theatrical demonstration of homeostasis long before Cannon coined the word. In 1775, Blagden spent 45 minutes in a chamber heated to 127°C (260°F)—along with a dog, a beefsteak, and some research associates. Being alive and capable of evaporative cooling, the dog panted and the men sweated. The beefsteak, being dead and unable to maintain home-ostasis, was cooked.

To take another example, a rise in blood pressure is sensed by stretch receptors in the wall of the heart and the major arteries above it. These receptors send nerve signals to a cardiac center in the brainstem. The cardiac center integrates this input with other information and sends nerve signals back to the heart to slow it and lower the blood pressure. Thus we can see that homeostasis is maintained by self-correcting negative feedback loops. Many more examples are found throughout this book.

It is common, although not universal, for feedback loops to include three components: a receptor, an integrator, and an effector. The receptor is a structure that senses a change in the body, such as the stretch receptors that monitor blood pressure. The integrating (control) center, such as the cardiac center of the brain, is a mechanism that processes this information, relates it to other available information (for example, comparing what the blood pressure is with what it should be), and "makes a decision" about what the appropriate response should be. The effector, in this case the heart, is the structure that carries out the response that restores homeostasis. The response, such as a lowering of the blood pressure, is then sensed by the receptor, and the feedback loop is complete.

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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

  • beau
    What negates change to maintain a set point?
    7 years ago
  • dehab girma
    How does positive and negative feedback mechanisms effect homeostatis?
    3 months ago

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