What causes the heart valves to open and close during the cardiac cycle?

• A. Electrical impulses from SA node
• B. Nerve impulses
• C. Pressure differences (blood pressure) driven by heart chamber contractions
• D. Hormonal signals

Answer: (C)

Valves open and close in response to pressure differences between the chambers and the vessels, which are created as the heart muscle contracts and relaxes. The electrical system times the contractions, but the actual valve movement is mechanical and driven by pressure gradients.

• When the ventricles contract, their pressure rises above the atria, closing the atrioventricular valves to prevent backflow and, if ventricular pressure exceeds arterial pressure, opening the semilunar valves to eject blood.

• As the ventricles relax and pressure falls, the semilunar valves close when arterial pressure becomes greater than ventricular pressure, and the atrioventricular valves open again when atrial pressure rises above ventricular pressure to fill the ventricles.

So the rhythm of valve opening and closing is governed by how pressure shifts during the cardiac cycle, not by nerve impulses or hormones acting directly on the valves.