What events define isovolumetric contraction and isovolumetric relaxation in the cardiac cycle?

• A. Isovolumetric contraction occurs when the ventricles contract with all heart valves closed, causing pressure to rise without volume change.
• B. Isovolumetric contraction occurs when the ventricles relax with all valves closed, causing pressure to fall without volume change.
• C. Isovolumetric contraction occurs when the ventricles contract with AV valves open; isovolumetric relaxation occurs with semilunar valves closed.
• D. Isovolumetric contraction occurs during atrial contraction; isovolumetric relaxation during atrial relaxation.

Answer: (A)

Isovolumetric contraction and isovolumetric relaxation happen when the ventricles are changing pressure but not volume because all the heart valves are closed.

During isovolumetric contraction, the ventricles begin to contract and push pressure up quickly. The atrioventricular valves (mitral and tricuspid) have already closed to prevent backflow from the atria, and the semilunar valves (aortic and pulmonary) are still closed, so no blood leaves or enters the ventricles. The result is a rise in ventricular pressure with no change in ventricular volume. This phase ends when the pressure inside the ventricles becomes higher than the pressure in the great arteries, causing the semilunar valves to open and ejection to begin.

During isovolumetric relaxation, the ventricles begin to relax after a contraction. The semilunar valves close first, preventing backflow from the aorta or pulmonary artery, and the AV valves remain closed for a brief moment as the ventricles continue to relax. With all valves closed again, the ventricular pressure falls but the volume remains the same. This phase continues until the ventricular pressure drops enough for the AV valves to open and ventricular filling resumes.

These phases are defined by the valve status (all valves closed) and the lack of volume change, even though pressure is changing. The other descriptions would imply valves are not all closed or that blood volume in the ventricles is changing, which would not be isovolumetric.