What event occurs in the heart as the atrioventricular (AV) valves are closing?

• A. The ventricles are relaxing and filling with blood.
• B. The atria are relaxing and filling with blood.
• C. The semilunar valves are opening to allow blood into the arteries.
• D. The ventricles begin to contract, marking the start of ventricular systole.

Answer: (D)

Verified step by step guidance

1. Step 1: Understand the function of the atrioventricular (AV) valves. These valves are located between the atria and ventricles and prevent backflow of blood into the atria when the ventricles contract.
2. Step 2: Recall the phases of the cardiac cycle. The heart undergoes systole (contraction) and diastole (relaxation). Ventricular systole refers to the contraction of the ventricles, which forces blood into the arteries.
3. Step 3: Recognize the sequence of valve actions during ventricular systole. As the ventricles begin to contract, the pressure inside them increases, causing the AV valves to close to prevent backflow into the atria.
4. Step 4: Note the relationship between the AV valves and semilunar valves. When the AV valves close, the semilunar valves (located at the exits of the ventricles) open to allow blood to flow into the arteries.
5. Step 5: Conclude that the closing of the AV valves marks the start of ventricular systole, as the ventricles begin to contract and pump blood into the arteries.