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CHAPTER 40 AN INTRODUCTION TO ANIMAL STRUCTURE AND FUNCTION Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Section C: Regulating the Internal Environment 1.Mechanisms of homeostasis moderate changes in the internal environment 2. Homeostasis depends on feedback circuits
More than a century ago, Claude Bernard made the distinction between external environments surrounding an animal and the internal environment in which the cells of the animal actually live. The internal environment of vertebrates is called the interstitial fluid. This fluid exchanges nutrients and wastes with blood contained in microscopic vessels called capillaries. 1. Mechanisms of homeostasis moderate changes in the internal environment Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Bernard also recognized that many animals tend to maintain relatively constant conditions in their internal environment, even when the external environment changes. While a pond-dwelling hydra is powerless to affect the temperature of the fluid that bathes its cells, the human body can maintain its “internal pond” at a more-or-less constant temperature of about 37 0 C. Similarly, our bodies control the pH of our blood and interstitial fluid to within a tenth of a pH unit of 7.4. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
There are times during the course of the development of an animal when major changes in the internal environment are programmed to occur. For example, the balance of hormones in human blood is altered radically during puberty and pregnancy. Still, the stability of the internal environment is remarkable. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Today, Bernard’s “constant internal milieu” is incorporated into the concept of homeostasis, which means “steady state,” or internal balance. Actually the internal environment of an animal always fluctuates slightly. Homeostasis is a dynamic state, an interplay between outside forces that tend to change the internal environment and internal control mechanisms that oppose such changes. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Any homeostatic control system has three functional components: a receptor, a control center, and an effector. The receptor detects a change in some variable in the animal’s internal environment, such as a change in temperature. The control center processes the information it receives from the receptor and directs an appropriate response by the effector. 2. Homeostasis depends on feedback circuits Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
One type of control circuit, a negative-feedback system, can control the temperature in a room. In this case, the control center, called a thermostat, also contains the receptor, a thermometer. When room temperature falls, the thermostat switches on the heater, the effector. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 40.9a
In a negative-feedback system, a change in the variable being monitored triggers the control mechanism to counteract further change in the same direction. Owing to a time lag between receptor and response, the variable drifts slightly above and below the set point, but the fluctuations are moderate. Negative-feedback mechanisms prevent small changes from becoming too large. Most homeostatic mechanisms in animals operate on this principle of negative feedback. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Our own body temperature is kept close to a set point of 37 o C by the cooperation of several negative- feedback circuits that regulate energy exchange with the environment. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 40.9b
One mechanism by which humans control body temperature involves sweating as a means to dispose of metabolic heat and cool the body. If the thermostat in the brain detects a rise in the temperature of the blood above the set point, it sends nerve impulses directing sweat glands to increase their production of sweat. This lowers body temperature by evaporative cooling. When body temperature drops below the set point, the thermostat in the brain stops sending the signals to the glands and the body retains more of the heat produced by metabolism. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
In contrast to negative feedback, positive feedback involves a change in some variable that trigger mechanisms that amplify rather than reverse the change. For example, during childbirth, the pressure of the baby’s head against sensors near the opening of the uterus stimulates uterine contractions. These cause greater pressure against the uterine opening, heightening the contractions, which cause still greater pressure. Positive feedback brings childbirth to completion, a very different sort of process from maintaining a steady state. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
While some aspects of the internal environment are maintained at a set point, regulated change is essential to normal body functions. In some cases, the changes are cyclical, such as the changes in hormone levels responsible for the menstrual cycle in women. In other cases, a regulated change is a reaction to a challenge to the body. For example, the human body reacts to certain infections by raising the set point for temperature to a slightly higher level, and the resulting fevers helps fight infection. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Over the short term, homeostatic mechanisms can keep a process, such a body temperature, close to a set point, whatever it is at that particular time. But over the longer term, homeostasis allows regulated change in the body’s internal environment. Internal regulation is expensive and animals use a considerable portion of their energy from the food they eat to maintain favorable internal conditions. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
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