At the beginning of diastole, intraventricular pressure is just below aortic pressure (the aortic
semilunar valve has just closed). Ventricular pressure continues to decrease without any change in volume (
isovolumetric relaxation), until pressure within the left ventricle is less than pressure within the left atrium. Since the movement of blood within the cardiovascular system is driven by pressure gradients, once pressure is greater in the atrium than in the ventricle, the bicuspid valve opens and blood flows into the ventricle. Thus, ventricular volume begins to increase. This continues until, toward the end of
diastole, the left atrium contracts slightly, increasing ventricular pressure as the final volume of blood is moved from the left atrium to the left ventricle. This marks the end of diastole and the beginning of systole. Systole is initiated by ventricular contraction, which increases pressure within the ventricle, eventually causing the bicuspid valve to close. The contraction within the ventricle causes pressure to increase without inducing any change in volume ([[isovolumetric contraction[[). Isovolumetric contraction continues until the pressure within the ventricle is greater than within the aorta (which has been decreasing since the beginning of diastole), leading to the opening of the aortic semilunar valve. Once the valve opens, blood begins to leave the ventricle, thereby decreasing its volume. This decrease in volume continues until the heart begins to relax, at which point pressure within the ventricle decreases below aortic pressure, and the aortic semilunar valve closes and diastole begins once again.
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