Biological Theories in the Domain of Physiology: Stability and Transformation as Two Governing Principles

Physiology has produced a rich theoretical foundation that applies to all known life forms, from microbes, to plants and animals, including humans. Compared with evolutionary theories, physiological theories have received much less attention and critical analysis from biologists and philosophers. Physiology includes many theories that are “local,” applying to particular sub-disciplines or organ systems. An overarching theory of physiology is homeostasis, first offered by Claude Bernard, and ultimately named and illustrated by Walter Cannon. Homeostasis conveys that life relies on a “stability of the internal milieu” to persist in face of an incompatible external environment. Various authors have offered clarifications, amendments, and outright replacement concepts for homeostasis. These alternatives have been motivated in part by the constrained notion that homeostasis does not admit changes that are readily observable in responses to normal environmental challenges and diseases or stresses. A less constrained view of homeostasis accommodates those concerns. Physiology includes processes that control transformative changes that homeostasis was never intended to address. These are generally reproductive, growth, and developmental transformations, and associated specializations such as migrations and metamorphoses. Evolution has generated a wide variety of such transformations, all of which must be implemented by physiological mechanisms. Some, such as metamorphosis, are non-ambiguous examples that are not accounted for by homeostasis alone. A new theoretical framework, named kinorhesis here, is proposed to encompass those transformations and the non-homeostatic processes that control them. The theoretical foundations of physiology deserve greater attention not only in the professions, but also in biology education. There is an imbalance created by teaching the relevance of physical sciences to biology without emphasizing the primacy of biological theories, including theories in evolution and physiology, which are not based in physics or chemistry and could not be deduced from any amount of physical-science knowledge.