Final answer:
Valves in the heart close during the isovolumetric phases of the cardiac cycle to prevent blood backflow. Atrioventricular valves close during isovolumetric contraction when ventricular pressure exceeds atrial pressure, and semilunar valves close during isovolumetric relaxation when ventricular pressure drops below arterial pressure.
Step-by-step explanation:
The valves in the heart close at different times during the cardiac cycle, corresponding to different phases of heart activity as illustrated in a Wiggers diagram. Specifically, the valves close:
- D. During the isovolumetric contraction phase: The atrioventricular valves close. This occurs after the atria have contracted and filled the ventricles with blood. As ventricular pressure rises sharply and surpasses atrial pressure, the tricuspid and mitral valves shut to prevent backflow into the atria.
- D. During the isovolumetric relaxation phase: The semilunar valves close. After the ventricles contract and eject blood into the aorta and pulmonary trunk, ventricular pressure falls. When ventricular pressure drops below the pressure in these arteries, the aortic and pulmonary valves close to prevent blood from flowing back into the heart's ventricles.
It is important to note that valves close to prevent backflow of blood and maintain one-way blood flow through the heart. The isovolumetric contraction and relaxation phases are critical since they involve no change in blood volume within the ventricles; they are constantly maintaining the ventricular volume by momentarily closing both sets of valves.