Heart sound
During each Heartbeat, Two sounds can be distinguished when the thorax is ausculted with a stethoscope.
These heart sounds, often described as lub-dup, are associated with closing of Heart valves.
First heart tone or sound S2 "sub" is caused by closure of aortic and pulmonary valves at the end of ventricular systole.
As the left ventricle empties, it's pressure falls below the pressure in the aorta and the aortic valves closes.
Similarly, as the pressure in right ventricle falls below the pressure in pulmonary artery, the pulmonary valve closes.
The first sound (S1) tends to louder, longer, than the second sound (S2).
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