Page 2, Cardiac Cycle, Dr. D. Penney

Introduction (continued....... ):

Atrial systole serves to "top off" ventricular filling. It is of minor importance at rest. See Figure 1.02 As the ventricle begins to contract, the AV valve closes; the tricuspid valve for the right heart, the mitral (bicuspid) valve for the left heart. Pressure then increases rapidly and the shape of the ventricle changes; however, volume remains constant. When ventricular pressure exceeds aortic pressure the semilunar valve opens, the pulmonic valve for the right heart, the aortic valve for the left heart. Flow now begins from the ventricle to the aorta, very rapid during early ejection, and much slower later during ejection. Ventricular volume falls rapidly. Also, although pressure in the ventricle soon falls below aortic, ejection continues to occur, indicating that pressure is not the only form of energy responsible for blood flow. Kinetic (velocity) energy acts in this case to keep the valve open during late ejection. The role of kinetic energy in this phenomenon is considered in number 3 in the Misconceptions List

After closure of the aortic valve, the ventricle begins to relax, pressure falling and shape changing, but again, this involves no change in volume. As ventricular pressure falls below atrial pressure, the AV valve opens and blood flows rapidly into the ventricle increasing its volume - the rapid filling phase. Mid-ventricular diastole is referred to as diastasis, wherein little change in volume and pressure occurs. This is followed by atrial systole, beginning another cycle.

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