The cardiac cycle refers to the sequence of events that occur during one heartbeat. The contraction phase of a cardiac cycle is known as systole (sis ‘-to-le); the relaxation phase is called diastole (di ‘-as-to-le). Note that the ventricles are relaxed when the atria contract, and the atria are relaxed when the ventricles contract. Systole increases blood pressure within a chamber, while diastole decreases blood pressure within a chamber.
When both the atria and ventricles are relaxed between beats, blood flows passively into the atria from the large veins leading to the heart and then passively into the ventricles. Then, the atria contract (atrial systole), forcing more blood into the ventricles so that they are filled. Immediately thereafter, the ventricles contract. Ventricular systole produces high blood pressure within the ventricles, which causes both AV valves to close and both semilunar valves to open. Opening of the semilunar valves allows blood to move into the arteries leading from the heart. Ventricular diastole immediately follows and the decrease in ventricle pressure allows the AV valves to open. Simultaneously, the semilunar valves close because of the greater blood pressure within the arteries. The cardiac cycle is then repeated.
The sounds of the heartbeat are usually described as lub- dup (pause) lub-dup, and so forth. These sounds are produced by the closing of the heart valves. The first sound results from the closing of the AV valves in the beginning of ventricular systole. The second sound results from the closing of the semilunar valves in the beginning of ventricular diastole. If any of the heart valves are defective and do not close properly, an additional sound, known as a heart murmur, may be heard.