What occurs during isovolumetric ventricular contraction?

• A. Ventricles fill with blood.
• B. Pressure increases in ventricles while volume remains constant.
• C. Blood is ejected from the ventricles.
• D. Atria contract.

Answer: (B)

The key idea is what happens in the ventricles during the brief burst of systole before blood actually moves out. In isovolumetric contraction, the ventricles start to thicken their walls as they contract, but both the inlet (mitral/tricuspid) and outlet (aortic/pulmonic) valves are closed. That means no blood can enter or leave, so the volume inside the ventricles stays the same. At the same time, the ventricular pressure rises rapidly as the muscle squeezes, until it becomes high enough to open the semilunar valves and begin ejection. So the correct statement is that pressure increases in the ventricles while volume remains constant.

Ventricles filling occurs during diastole when the heart is relaxed and the AV valves are open. Atria contracting is a separate event—the atria squeeze blood into the ventricles just before ventricular systole. Blood is ejected from the ventricles only after the semilunar valves open, which happens after isovolumetric contraction.