At start of ventricular contraction:

Explanation

At the start of ventricular contraction, the biventricular heart valves (mitral and tricuspid valves) are closed and the semilunar valves (aortic and pulmonic valves) are opened. This allows the ventricles to contract and pump blood out of the heart. The mitral valve connects the left atrium to the left ventricle, and the tricuspid valve connects the right atrium to the right ventricle. When these valves are closed, they prevent blood from flowing back into the atria during ventricular contraction. The aortic valve connects the left ventricle to the aorta, and the pulmonic valve connects the right ventricle to the pulmonary artery. When these valves are opened, they allow blood to flow out of the ventricles into the aorta and pulmonary artery. The opening and closing of the heart valves is controlled by the electrical activity of the heart.