What is happening in the heart as the semilunar valves are closing?

• A. The atria are relaxing and the semilunar valves are opening.
• B. The atria are contracting and filling the ventricles with blood.
• C. The ventricles are relaxing and the atrioventricular valves are opening.
• D. The ventricles are contracting and ejecting blood into the arteries.

Answer: (C)

Verified step by step guidance

1. Understand the function of the semilunar valves: These valves are located between the ventricles and the major arteries (aorta and pulmonary artery). They prevent backflow of blood into the ventricles after contraction.
2. Identify the cardiac cycle phase: The semilunar valves close at the end of ventricular systole, which is when the ventricles have finished contracting and are beginning to relax.
3. Recognize the role of the atrioventricular (AV) valves: As the ventricles relax, the pressure within them drops, causing the AV valves (tricuspid and mitral) to open, allowing blood to flow from the atria into the ventricles.
4. Differentiate between systole and diastole: During systole, the ventricles contract and eject blood, causing the semilunar valves to open. During diastole, the ventricles relax, leading to the closure of the semilunar valves and opening of the AV valves.
5. Conclude the sequence of events: As the semilunar valves close, the ventricles are in the early phase of diastole, relaxing and preparing to fill with blood from the atria through the open AV valves.