A physiologist's view of homeostasis

Abstract

Homeostasis is a core concept necessary for understanding the many regulatory mechanisms in physiology. Claude Bernard originally proposed the concept of the constancy of the "milieu interieur," but his discussion was rather abstract. Walter Cannon introduced the term "homeostasis" and expanded Bernard's notion of "constancy" of the internal environment in an explicit and concrete way. In the 1960s, homeostatic regulatory mechanisms in physiology began to be described as discrete processes following the application of engineering control system analysis to physiological systems. Unfortunately, many undergraduate texts continue to highlight abstract aspects of the concept rather than emphasizing a general model that can be specifically and comprehensively applied to all homeostatic mechanisms. As a result, students and instructors alike often fail to develop a clear, concise model with which to think about such systems. In this article, we present a standard model for homeostatic mechanisms to be used at the undergraduate level. We discuss common sources of confusion ("sticky points") that arise from inconsistencies in vocabulary and illustrations found in popular undergraduate texts. Finally, we propose a simplified model and vocabulary set for helping undergraduate students build effective mental models of homeostatic regulation in physiological systems.

Keywords: core concepts; homeostasis; negative feedback; regulation.

Fig. 1.

Diagram of a generic homeostatic regulatory system. If the value of the regulated variable is disturbed, this system functions to restore it toward its set point value and, hence, is also referred to as a negative feedback system.

Fig. 2.

Simplified representation of a homeostatic regulatory system. Several components shown in Fig. 1 are combined in this representation. The reader should refer to Table 1 to find correspondence between components of physiologically significant homeostatic regulatory systems and this simplified representation. For example, chemosensors in the carotid bodies and aortic body are “sensors,” the brain stem is the “control center,” and the diaphragm and other respiratory muscles are “effectors” in the homeostatic regulatory system for arterial Po₂.