1103 Chapter 1. An Introduction to the Human Body
1.5 Homeostasis
Learning Objectives
By the end of this section, you will be able to:
- Define the term “homeostasis”
- Explain the importance of homeostasis
- Specify the four main variables that must be regulated to maintain homeostasis
- Define “internal environment” and explain its relationship to the following: extracellular fluid, intracellular fluid, plasma, interstitial fluid, lymph
- Define the term “stressor” or “stimulus”
- Provide two examples of internal stressors/stimuli
- Provide two examples of an external stressors/stimuli
- Define a feedback loop
- Explain what is meant by negative and positive feedback systems and describe their role in homeostasis
- Describe the role of the skin in monitoring body temperature as an example of a negative feedback system
Homeostasis refers to a relatively stable set of conditions within an organism’s internal environment. Within the human body, maintaining a healthy environment for living cells requires maintaining appropriate conditions in the extracellular fluids – interstitial fluid and blood plasma – for each living cell to be able to function properly.
Maintaining homeostasis requires that the body continuously monitor its internal conditions. From body temperature to blood pressure to levels of certain nutrients, each physiological condition has a particular set point. A set point is the physiological value around which the normal range fluctuates. A normal range is the restricted set of values that is optimally healthful and stable. For example, the set point for normal human body temperature is approximately 37°C (98.6°F) Physiological parameters, such as body temperature and blood pressure, tend to fluctuate within a normal range a few degrees above and below that point. Control centers in the brain and other parts of the body monitor and react to deviations from homeostasis using negative feedback. Negative feedback is a mechanism that reverses a deviation from the set point. Therefore, negative feedback maintains body parameters within their normal range. The maintenance of homeostasis by negative feedback goes on throughout the body at all times, and an understanding of negative feedback is thus fundamental to an understanding of human physiology.
Negative Feedback
A negative feedback system has three basic components (Figure 1a). A sensor, also referred to a receptor, is a component of a feedback system that monitors a physiological value. This value is reported to the control center. The control center is the component in a feedback system that compares the value to the normal range. If the value deviates too much from the set point, then the control center activates an effector. An effector is the component in a feedback system that causes a change to reverse the situation and return the value to the normal range.
In order to set the system in motion, a stimulus must drive a physiological parameter beyond its normal range (that is,