What is the consequence of the AV valves closing at the onset of systole?

What is the consequence of the AV valves closing at the onset of systole?

• A. They open to allow atrial contraction.
• B. They close to prevent backflow into the atria.
• C. They close to prevent backflow into the ventricles.
• D. They open during systole to begin ejection.

Answer: (B)

When the ventricles start to contract, their pressure rises above the atria, causing the atrioventricular valves (between the atria and ventricles) to snap shut. This closure prevents blood from flowing backward into the atria as the ventricles contract, and it also creates the first heart sound (S1). The valves stay closed during isovolumetric contraction until ventricular pressure becomes high enough to open the semilunar valves for ejection.

When the ventricles start to contract, their pressure rises above the atria, causing the atrioventricular valves (between the atria and ventricles) to snap shut. This closure prevents blood from flowing backward into the atria as the ventricles contract, and it also creates the first heart sound (S1). The valves stay closed during isovolumetric contraction until ventricular pressure becomes high enough to open the semilunar valves for ejection.