Heart – Systole and Diastole

During the cardiac cycle, two contractions occur.  Systole occurs when the muscles contract and diastole occurs when the muscles relax.   Atrial systole occurs when the atria contract and push blood into the lower ventricles.  During this point, the aortic valve is closed.   Ventricular contraction then occurs when the much larger muscles of the ventricles contract and push blood out of the aorta and the pulmonary arteries.   The left ventricle is much larger and provides enough thrust for blood to leave the heart and travel through the body via the aorta.  The right ventricle only needs to push blood to the lungs and back via the pulmonary arteries and veins.  This is why the left side of the heart is more muscular than the right and why we tend to feel our heartbeat on the left side of our chest.

animated heartWhen listening to heart sounds with a stethoscope, the characteristic “lub-dub” noises occur when the valves snap shut.   The atrioventricular valves open during atrial systole so that blood moves into the ventricles.  These valves then close during ventricular systole when the aortic valve opens to allow blood to leave the heart.   Leaking valves are often heard as a “psshh” sound at the hind end of the lub-dub.

The University of Washington has an excellent page that displays heart sounds and common disorders, such as an atrial septal defect, or a heart murmur.

heart sounds