Chapter 2: Control of the Internal Environment

OBJECTIVES

By studying this chapter, you should be able to do the following:

1. Define the terms homeostasis and steady state.

2. Diagram and discuss a biological control system.

3. Give an example of a biological control system.

4. Explain the term negative feedback.

5. Define what is meant by the gain of a control system.

OUTLINE

• Homeostasis: Dynamic Constancy 31

• Control Systems of the Body 33

• Nature of the Control Systems 33

  • Negative Feedback 34

  • Positive Feedback 34

  • Gain of a Control System 34

• Examples of Homeostatic Control 34

  • Regulation of Body Temperature 34

  • Regulation of Blood Glucose 35

• Exercise: A Test of Homeostatic Control 35

• Exercise Improves Homeostatic Control via Cellular Adaptation 35

• Stress Proteins Assist in the Regulation of Cellular Homeostasis 37

KEY TERMS

acclimation

adaptation

autocrine signaling

biological control system

cell signaling

control center

effectors

endocrine signaling

endocrine system

gain

heat shock proteins

homeostasis

intracrine signaling

juxtacrine signaling

negative feedback

paracrine signaling

sensor

steady state

stress proteins

INTRODUCTION

More than 100 years ago, the French physiologist Claude Bernard observed that the “milieu interior” (internal environment) of the body remained remarkably constant despite a changing external environment (see A Look Back—Important People in Science). The fact that the body maintains a relatively constant internal environment in spite of stressors (e.g., heat, cold, exercise) is not an accident but is the result of many control systems (15). Control mechanisms that are responsible for maintaining a stable internal environment constitute a major chapter in exercise physiology, and it is helpful to examine their function in light of simple control theory. Therefore, this chapter introduces the concept of “control systems” and discusses how the body maintains a relatively constant internal environment during periods of stress. However, before you begin to read this chapter, take time to review Research Focus 2.1. This box provides an overview of how to interpret graphs and gain useful information from these important tools of science.