Most physiological processes are nonlinear. Nonlinear behaviors such as threshold, saturation, and polynomial/exponential dynamics heavily influence clinical decision-making. Further, the human mind tends to default to linear models of causation and change. Linearity means that an incremental change to an input variable leads to a proportional increase in an output variable. The proportionality is insensitive to both the starting value of the input variable and the size of the incremental change. Linearity also implies that, for systems with more than one input variable, the output is exclusively dependent on additive interactions among the input variables. With this definition, it is clear that nonlinearity abounds in physiology. Basic processes such as oxygen-hemoglobin dissociation and cardiac functional reserve are wholly nonlinear. At its core, physiology violates “intuitive” linear mental models of cause-and-effect.
Michael A. Choma, MD, PhD