Question

During ventricular systole, which valves are open?

• A. AV valves
• B. Semilunar valves
• C. Both AV and semilunar valves
• D. All valves closed

Hint:

There is a very brief moment at the beginning of systole called "isovolumetric contraction" where all valves are closed, but the "open" state during systole refers to the ejection phase.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
Ventricular systole is the phase of the cardiac cycle when the ventricles contract to pump blood out of the heart into the pulmonary and systemic circulation.

Step 2: Detailed Explanation:
The sequence during ventricular systole is as follows:
1. As the ventricles start contracting, pressure rises rapidly.
2. To prevent blood from flowing back into the atria, the Atrioventricular (AV) valves snap shut.
3. Once the pressure in the ventricles exceeds the pressure in the aorta and pulmonary artery, the Semilunar valves are forced open.
4. This allows the oxygenated blood to leave the left ventricle via the aorta and deoxygenated blood to leave the right ventricle via the pulmonary artery.

Step 3: Final Answer:
During the active ejection phase of ventricular systole, the semilunar valves are open.

Answer 2

This can be worked out by directly comparing ventricular pressure against atrial pressure and against arterial pressure during the ejection part of systole, and checking each valve option against those comparisons.

1. AV valves: During systole, ventricular pressure is far higher than atrial pressure, and a valve only stays open when pressure downstream is lower than pressure upstream in the direction of flow it guards. Since the AV valves are meant to stop backflow into the atria, this high ventricular to low atrial pressure difference forces them shut, so AV valves being open is inconsistent with systolic pressures.
2. Semilunar valves: Once ventricular contraction raises ventricular pressure above the pressure in the aorta and pulmonary artery, the semilunar valves are pushed open, allowing forward flow into the arteries. This pressure relationship, ventricle higher than artery, holds throughout the ejection phase of systole, matching semilunar valves being open.
3. Both AV and semilunar valves: For both sets to be open at once, ventricular pressure would need to be simultaneously lower than atrial pressure and higher than arterial pressure, an impossible combination during a phase where ventricular pressure is climbing well above both.
4. All valves closed: All four valves are closed together only very briefly, during the isovolumic contraction phase right before ejection begins, once ejection is underway and blood is actually being pumped out, the semilunar valves must be open for that flow to happen.

Comparing ventricular pressure with atrial and arterial pressure throughout systole shows the semilunar valves are the ones held open during ejection.

So the correct answer is Semilunar valves.