The Donders model of the circulation in normo- and pathophysiology

A model of the closed human cardiovascular loop is developed. This model, using one set of 88 equations, allows variations from normal resting conditions to exercise, as well as to the extreme condition of a circulation following cardiac arrest. The principal purpose of the model is to evaluate the continuum of physiological conditions to cardiopulmonary resuscitation effects within the circulation.  

Within the model, Harvey's view of the circulation has been broadened to include impedance-defined flow as a unifying concept. The cardiac function curve, the relation between ventricular filling and output, changes during exercise. First, it rotates counterclockwise and stretches along the output axis, second, it shifts along the filling axis. The first is induced by sympathetic control, the second by respiratory control. The model shows that depth of respiration, sympathetic stimulation of cardiac contractile properties and baroreceptor activity can exert powerful influences on the increase in cardiac output, while heart and respiratory rate increases tend to exert an inhibiting influence. Impedance-defined flow encompasses both positive and negative effects.

The model demonstrates the limitations to cardiopulmonary resuscitation caused by external force applied to intrathoracic structures, with effective cardiac output being limited by collapse and sloshing. It demonstrates that the clinical inclination to apply high pressures may be unjustified.