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During which phases of the cardiac cycle are all the valves closed?
The cardiac cycle consists of seven phases that lead to blood being circulated throughout the body. The phases of the cardiac cycle are atrial systole, isovolumetric ventricular contraction, rapid ventricular ejection, reduced ventricular ejection, isovolumetric ventricular relaxation, rapid ventricular filling, and reduced ventricular filling. The isovolumetric ventricular contraction phase is marked by an increase in ventricular pressure as the ventricles contract. This phase begins when the atrioventricular (tricuspid and mitral) valves close due to the pressure in the ventricles exceeding the pressure in the atria. The volume of the blood in the ventricles remains constant during isovolumetric ventricular contraction because the atrioventricular and semilunar valves (aortic and pulmonic) are closed. Isovolumetric ventricular relaxation occurs between ventricular ejection of blood and ventricular filling. It begins when the semilunar valves close following ventricular contraction and ends with the opening of the atrioventricular valves. The volume of blood in the ventricles remains constant because each of the valves is closed, and the pressure decreases because the ventricles are relaxed.
Atrial systole and isovolumetric ventricular contraction (A) is incorrect because, during atrial systole, the atrioventricular valves are open, as blood is flowing into the ventricles. Rapid ventricular filling and reduced ventricular filling (C) are phases of the cardiac cycle where the atrioventricular valves are open and the semilunar valves are closed. Reduced ventricular ejection and isovolumetric ventricular relaxation (D) is incorrect because the semilunar valves are open during reduced ventricular ejection.
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Q: In which part of the electrocardiogram does isovolumetric ventricular contraction begin?Reveal Answer
A: QRS complex, which represents the electrical activation of the ventricles.