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A Brief & Easy Explanation of The Cardiac Cycle

The cardiac cycle involves all the activities of the heart through one complete heartbeat, according to Cliffsnotes.com. The cycle involves the ventricles, which are two lower chambers of the heart that receive blood from two smaller heart chambers called atria, according to MedicineNet.com. The cardiac cycle also includes two major phases: the systolic phase and the diastolic phase. The systolic phase refers to the period when the ventricles and atria of the heart contract, while the diastolic phase is the period between contractions when the ventricles relax and fill with blood.

  1. Relaxation

    • During the relaxation period, the ventricles and two types of valves---atrioventricular (AV) and semilunar valves---remain closed. The AV valves allow blood to flow from the atria to the ventricles, while the semilunar valves prevent blood from flowing back into the heart, according to WordNet. The ventricles' volume remains unchanged during the relaxation period.

    Rapid Filling

    • The AV valves open, and deoxygenated blood fills the ventricles from the atria. The first step of the filling process is rapid filling, which occurs as blood flows into the empty and relaxed ventricles from the atria, thus allowing the ventricles to increase rapidly in volume. The ventricles remain in the diastolic phase during this period.

    Slower Filling

    • A slower filling event then occurs because most of the volume of the ventricles already is occupied by blood. The atria contract and force the remaining blood from the atria into the ventricles, according to the University of Utah. The blood volume at the end of this period is called the end-diastolic volume.

    Contraction

    • The ventricles then contract. The AV valves are forced shut and at a brief moment the semilunar valves also are closed, causing the volume of the ventricles to remain unchanged. The continuing contraction of the ventricles increases the pressure in the ventricles and force the semilunar valves open. Blood then is forced out of the ventricles.

    Relaxation

    • The ventricles then begin to relax, and the blood in the aorta and pulmonary trunk begin to flow backward, causing the semilunar valves to close. The pulmonary trunk divides into arteries that carry deoxygenated blood to the lungs, while the aorta is the main artery that carries blood from the heart to the rest of the body, according to the National Institutes of Health. The closing of the semilunar valves causes a small boost in blood pressure. The amount of blood that remains in the ventricles during this period is called the end-systolic volume.

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