The 10th J. A. F. Stevenson Memorial Lecture Outline of a physical theory of physiological systems
Authors: Yates F. Eugene 
Published: April 2011

I examine the scientific status of "organismic biology" and find it to be weak and vitalistic. I propose that we now need integrative theories based on a physical approach to biology. Certain technical terms about systems, regulation and control, and the minimum requirements of a physical theory for an organism are defined. Homeostasis, the earlier first theory of physiological stability, is shown to be incomplete. I then introduce a physical theory for complex systems (homeokinetics) based upon statistical mechanics, nonlinear dynamics, and irreversible thermodynamics. The question of whether or not biological systems lie within the domain reachable by such physical analysis is considered in detail, and answered in the affirmative. I then give highlights of the physical theory to be invoked. Following the work of A. S. Iberall, I introduce a generalized stress tensor that shows in what way complex systems that internalize most of their transformations of energy can be examined from a statistical mechanical point of view, and, via a generalized Reynolds number, how a condensation into a cooperative state comes about. The bulk/shear viscosity ratio is shown to be a powerful index of complexity, and a measure of the tendency of quantized or modal action (energy × time) to replace momentum as a conserved quantity in a system. This ratio is a basis for generalizing the effects describable by the stress tensor. Restricting the illustrations to simple examples of potential–flux networks, I show principles of dynamic regulation as they apply to simple electrical circuits and simple hydraulic circuits. I present similar analyses of the pump and circuit characteristics of the human circulation, following Guyton, but emphasizing here that central circulatory pressures empower many of the body mechanisms, and the cardiac output specifies the supply in all locales. Advantages and shortcomings of the "equilibrium operating point" measure of regulation are discussed. I offer the examples of the regulation band on systolic, diastolic, and mean blood pressures as an indication of the wide range of behavior of dynamically regulated variables.The physical principles of homeokinetics are next applied to cardiovascular architecture and function, and (following Guyton and Iberall separately) I suggest that the special morphology of renal vessels dominates long-term cardiovascular pressure regulation. An Epilogue proposes that the physical principles set forth are the foundation for the design of physiological experiments, whenever regulatory behavior is at issue.The chief images provided are that mean state (first statistical moment) regulation occurs at every hierarchical level of a stable, complex system. The second statistical moment (variance) of the regulation can be rather large. The stability is nonlinear, and brought about by parametric modulation of multiple, coupled, thermodynamic, cyclic, dissipative engines within. Nonlinear oscillators are used as dynamic regulators, hooked up in circular causalities. Behavior is quantized into modal actions. Global stability is a trajectory through a set of the modes. Homeostasis is expanded to homeokinesis through the physical concept of dynamic regulation. Physiology then emerges as level-independent, physical biology.Feedback or control (as technologically defined) are both rare, and in any case, are not the source of the stability regime. They merely quicken responses.