There are different phases to a healthy heartbeat, or cardiac cycle. During the first phase, the heart is relaxed and blood
flows from the venae cavae Into the right atrium and from the pulmonary veins into the left atrium. The tricuspid and mitral valves open, then blood flows from the atria into the ventricles (Fig. 2.6A).
In the next phase, the atria contract and force more blood through the tricus-pid and mitral valves, which "tops up" the ventricles. This is called atrial systole (Fig. 2.6B).
Next, the right and left ventricles contract. This is called ventricular systole. The mitral and tricuspid valves shut and the pulmonary and aortic valves open as blood travels into the pulmonary artery and the aorta (Fig. 2.6C).
When the ventricles complete their contraction phase, the pulmonary and aortic valves close. The atria expand again and fill with blood. The ventricles relax and the tricuspid and mitral valves open, completing one cardiac cycle and beginning another. If the heart is beating sixty times per minute, all of this is accomplished in one second.
In an adult at rest, a heart rate of seventy beats per minute is fairly typical. If you're exercising, such as running, weightlifting, swimming, or playing tennis, your heart rate increases to supply more blood to your muscles, which need more oxygen and nutrients as they work.
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