One unique aspect of this textbook is the incorporation of a relatively sophisticated, real-time model of the cardiovascular system that is used to explain the dynamic interactions between heart and vasculature. Elements of the model pop up during Try it Now options in which specific parameter values can be varied and effects observed. In addition, the entire simulation can be called by rotating the iPad to the landscape position. So, exactly how is the model constructed?

Hydraulic networks (like the circulatory system) can be represented by electronic circuits because of the analogies between flow of fluids through pipes and flow of electrical currents through conducting wires, between pressure and voltage, between hydraulic and electrical resistance, and between hydraulic and electrical capacitance. One electrical representation of the cardiovascular system is depicted in the following Figure.

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.

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