valves

• During diastole, blood returning to the atria flows into the ventricles. In diastole the pressure in the arteries closes the semi-lunar valves.
• Atrial systole transfers the remainder of the blood through the atrio-ventricular (AV) valves to the ventricles.
• Ventricular systole closes the AV valves and pumps the blood out through the semi-lunar (SL) valves to the aorta and pulmonary artery.
• The opening and closing of the AV and SL valves are responsible for the heart sounds heard with a stethoscope.