What is the relationship between atrial pressure and ventricular pressure at the beginning of the cardiac cycle?

What is the relationship between atrial pressure and ventricular pressure at the beginning of the cardiac cycle?

• A. Atrial pressure is greater than ventricular pressure.
• B. Ventricular pressure is greater than atrial pressure.
• C. They are equal.
• D. Atrial pressure is zero.

Answer: (A)

At the start of the cycle, the atria contract to finish filling the ventricles. For blood to move from the atria into the ventricles, the atrial pressure must be higher than the ventricular pressure, so the AV valves are open and filling occurs. As soon as the ventricles begin to contract, ventricular pressure rises above atrial pressure, causing the AV valves to close and the cycle to progress. If the ventricular pressure were higher at the start, the AV valves wouldn’t open for atrial filling, and if pressures were equal or atrial pressure were zero, there wouldn’t be a driving force for filling.

At the start of the cycle, the atria contract to finish filling the ventricles. For blood to move from the atria into the ventricles, the atrial pressure must be higher than the ventricular pressure, so the AV valves are open and filling occurs. As soon as the ventricles begin to contract, ventricular pressure rises above atrial pressure, causing the AV valves to close and the cycle to progress. If the ventricular pressure were higher at the start, the AV valves wouldn’t open for atrial filling, and if pressures were equal or atrial pressure were zero, there wouldn’t be a driving force for filling.