The first cardiac sound is created when the mitral valve closes as a result of the isovolumetric contraction, which raises left ventricular pressure above atrial pressure.
The amount of blood inside the chamber stays constant since blood is not yet being expelled from the ventricles. As a result, the term "isovolumic contraction," sometimes known as "isovolumetric contraction," refers to this early stage of ventricular systole. The onset of systole's first phase, known as isovolumetric ventricular contraction (a-b), is signaled by the emergence of the QRS complex on the electrocardiogram and the closing of the AV valves at point (a)
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