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Unformatted text preview: The control center, which determines the level (set point) at which a variable is to be main- tained, analyzes the information it receives and then determines the appropriate response or course of action. The third component is the effector, which provides the means for the control centers re- sponse (output) to the stimulus. Information flows from the control center to the effector along the efferent pathway . ( Efferent information exits from the control center.) The results of the response then feed back to influence the stimulus, either by depressing it (negative feedback) so that the whole control mechanism is shut off or by enhancing it (positive feedback) so that the reaction continues at an even faster rate. Most homeostatic control mechanisms are negative feedback mechanisms. In such sys- tems, the net effect of the response to the stimulus is to shut off the original stimulus or reduce its in- tensity. A frequently used example of a negative feedback system is a home heating system con- nected to a thermostat. In this situation, the ther- mostat contains both the receptor and the control center. If the thermostat is set at 20C (68F), the heating system (effector) will be triggered ON when the house temperature drops below that set- ting. As the furnace produces heat, the air is warmed. When the temperature reaches 20C or slightly higher, the thermostat sends a signal to shut off the furnace. Your body thermostat, located in a part of your brain called the hypothalamus, oper- ates in a similar way to regulate body temperature. Other negative feedback mechanisms regulate heart rate, blood pressure, breathing rate, and blood levels of glucose, oxygen, carbon dioxide, and minerals. Because they tend to increase the original dis- turbance (stimulus) and to push the variable farther from its original value, positive feedback mechanisms are rare in the body. Typically these mechanisms control infrequent events that occur explosively and do not require continuous adjust- ments. Blood clotting and the birth of a baby are the most familiar examples of positive feedback mechanisms. Homeostatic Imbalance Homeostasis is so important that most dis- ease can be regarded as a result of its disturbance, a condition called homeostatic imbalance. As we age, our body organs become less efficient, and our internal conditions become less and less stable. These events place us at an increasing risk for ill- ness and produce the changes we associate with aging. Examples of homeostatic imbalance will be pro- vided throughout this book to enhance your under- standing of normal physiological mechanisms. These homeostatic imbalance sections are preceded by the symbol to alert you that an abnormal condition is being described. The Language of Anatomy Learning about the body is exciting, but our in- terest sometimes dwindles when we are con- fronted with the terminology of anatomy and physiology. Lets face it. You cant just pick up an anatomy and physiology book and read it as...
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This note was uploaded on 01/31/2011 for the course BIO 210 taught by Professor S during the Spring '10 term at University of Toronto- Toronto.
- Spring '10