During one cardiac cycle, all four heart chambers go through a contraction period, called systole, and a relaxation period, called diastole. As a result of the cyclic contraction and relaxation of the ventricles, blood pressure in the pulmonary and systemic circuits rises and falls.
Let us consider a complete cycle for the left side of the heart. Chamber pressures and volumes can be shown on a graph. Here, the red line traces the pressure inside the left ventricle. The green line shows pressure in the aorta, and the blue line traces the pressure within the left atrium.
We will begin our look at the cardiac cycle at the end of a cardiac cycle. At this point the atria and ventricles are in diastole, and the ventricles are roughly 70% filled with blood. The cardiac cycle begins with atrial systole.
As the left atrium contracts, blood remaining in the left atrium is forced into the left ventricle. The left atrium then relaxes, and atrial diastole begins. As the left atrium relaxes, the left ventricle contracts, and ventricular systole begins. Ventricular contraction increases blood pressure within the left ventricle. When left ventricular pressure exceeds left atrial pressure, the bicuspid valve shuts, preventing blood from flowing back into the atrium. Ventricular pressure continues to increase until it exceeds the blood pressure in the aorta. At this point, the aortic semilunar valve is forced open, and blood flows into the aorta.
When the left ventricle finishes contracting, it enters the period of ventricular diastole. Blood pressure in the left ventricle declines rapidly. When it falls below the aortic pressure, the semilunar valve closes and prevents backflow.
The left atrium remains in diastole throughout ventricular systole and continues throughout most of ventricular diastole. During ventricular diastole, venous blood enters the left atrium, and when blood pressure within the left atrium exceeds blood pressure in the left ventricle, the mitral valve opens and passive filling of the ventricle occurs. This continues until this cardiac cycle ends, and atrial systole marks the beginning of the next cycle.