Critique of a Large-Scale Organ System Model: Guytonian Cardiovascular Model

This paper reviews Guyton’s model which is large not only in the number of its components but also in the time scale that it spans. The evolution of this model is explained in three stages. Guyton started with a drastically simplified model of the entire cardiovascular system as a closed hydraulic loop. It accounted for short-term regulations of cardiac output with a special emphasis on the role of blood volume and the vascular capacity. Guyton’s research objective was then directed toward the analysis of longterm regulation of arterial pressure. Two slowly acting mechanisms were considered as particularly important: (1) the marked increase or decrease of urinary output with only slight increase or decrease in arterial pressure (the renal function curve in the Guytonian model) and (2) long-term vascular autoregulation which includes changes in the extent of vascularization as well as constriction or dilation of existing vessels to match the blood flow with the oxygen demand in tissues. This second-stage model explained the transient dynamics and steady equilibrium of renal hypertension. The current version of Guyton’s model incorporates a variety of additional endocrine and neural mechanisms which parametrically control the renal function curve. With the enormous growth, the identification (or estimation) capability of the model is bound to degrade while its use for multiple parameter sensitivity tests expands. The modeller’s group has attempted to minimize the hazards by frequent checks of model predictions with experimental studies. This interactive effort, plus their concern over these long-term regulatory mechanisms, make the Guytonian model a unique venture in modern cardiovascular physiology.