Systole is the phase of the cardiac cycle when both ventricles contract (Applegate, 2000, 251). 33.3 Diastole is the phase of the cardiac cycle when both ventricles relax.
33.4 The cardiac cycle refers to the alternating contraction and relaxation of the myocardium in the walls of the heart chambers: first the atria contract, then relax, and then the ventricles contract and relax. The cycle begins with atrial systole, when both atria contract together, and when they finish their contraction phase, the ventricles begin their contraction phase. The cycle ends when the ventricle relax, and the atria begin to contract again.
33.5 The AV valves are closed during ventricular systole, when the ventricle contract.
33.6 The AV valves are open during atrial systole, when the atria are contracting.
33.7 Relaxation of the ventricles causes the AV valve to open to allow blood flow from the atria to the ventricles.
33.8 Contraction of the ventricles causes the AV valves to close to prevent backflow of blood.
33.9 The semilunar valves are closed at the beginning of ventricular diastole.
33.10 The semilunar valves open in response to ventricular systole so blood can flow to the lungs from the right ventricle or to the body from the left ventricle.
33.11 The semilunar valves close in response to ventricular diastole so blood does not flow back into the ventricles.
33.13 Both sets of valves are closed at the beginning of ventricular diastole.
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