Physiological Redundancy and the Integrative Responses to Exercise.

The biological responses to acute and chronic exercise are marked by a high level of physiological redundancy that operates at various levels of integration, including the molecular, cellular, organ-system, and whole-body scale. During acute exercise, this redundancy protects whole-body homeostasis in the face of 10-fold or more increases in whole-body metabolic rate. In some cases, there are "trade-offs" between optimizing the performance of a given organ or system versus whole-body performance. Physiological redundancy also plays a key role in the adaptive responses to exercise training and high levels of habitual physical activity, including the positive effects of regular exercise on health. Appreciation of the general principles of physiological redundancy is critical to (1) gain an overall understanding of short- and long-term responses to exercise, and (2) place physiological responses occurring at various levels of integration in perspective.