[Physiology of the heart valves]

The function of healthy heart valves has been understood as the "on-off switch", which initiates and terminates blood flow in the cardiac cycle. It is believed that an impedance of the valves against blood flow is minimal and their closure is abrupt and complete. Clinically, an increase in impedance of the diseased valve is estimated by measuring pressure difference across that valve. However, the pressure gradient induced by acceleration of blood along the outflow tract of the ventricle or the artery during the early ejection phase, or along the inflow tract of the ventricle during the rapid filling phase can not be estimated separately from the pressure difference produced by valvular impedance in physiological state. Opening speed of the valve is related to the factors determining the velocity of blood flow. Mechanisms of a valve closure have been explained based on the Henderson and Johnson's concepts. Recent advances in ultrasonic technology can only confirm their concepts. Measurement of the pressure gradient across the semilunar valves in the late ejection phase has also several technical difficulties. Relationships among pressure, flow and valve motion have been extensively investigated, how-exact determination of the timing of the valve closure is not detected. The heart valves are considered to function as a passive organ throughout the cardiac cycle except for the atrioventricular valves including the subvalvular apparatus during systolic phase. The development of new methodology is necessary for the investigation of this field.