The atria and ventricles are represented by time-varying capacitances (shown by capacitors with the arrows through them). Each vascular system is represented by a series of resistances and capacitors. The heart valves are represented by diodes that permit flow in only one direction.

Using this model as a guide, a set of time varying, simultaneous differential equations can be derived to represent the circulatory system. These equations are solved in real-time and the results yield real time pressure, volume and flow tracings that form the basis of the simulations used in this textbook. More complex models of the cardiovascular system are available. However, it is remarkable how much circulatory physiology and pathophysiology this simple model explains. Of course, it is not necessary to have any understanding of electronic circuitry or differential equations to understand the remainder of this text; all explanations are based on descriptions of physiological principles without detailed reference to this model.

In addition to the basic model shown above, more complex situations can be modeled. Such models are used in the *full version of Harvi* to simulate cardiovascular disease states and mechanical circulatory support devices.