The heart can be classified as a simple reciprocating pump. The mechanical principles of a reciprocating pump are illustrated in Fig. 1. The pumping chamber has a variable volume and input and output ports. A one-way valve in the input port is oriented such that it opens only when the pressure in the input chamber exceeds pressure within the pumping chamber. Another one-way valve in the output port opens only when pressure in the pumping chamber exceeds the pressure in the output chamber. In the example, the rod and crankshaft cause the diaphragm to move back and
forth. The chamber's volume changes as the piston moves, causing the pressure within to rise and fall. In the heart, the change in volume is the result of contraction and relaxation of the cardiac muscle that makes up the ventricular walls. One complete rotation of the crankshaft in Fig. 1 will result in one pump cycle. Each cycle, in turn, consists of a filling phase and an ejection phase. The filling phase occurs as the pumping chamber's volume is increasing and drawing fluid through the input port. During the ejection phase, the pumping chamber's volume is decreasing and fluid is ejected through the output port. The volume of fluid ejected during one pump cycle is referred to as the stroke volume. The volume of fluid pumped each minute can be determined by simply multiplying the stroke volume times the number of pump cycles per minute.
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