The cardiac cycle
The cardiac cycle is a sequence of events that occur with every heartbeat. It can be divided into systole, which is when the heart contracts, and diastole, which is when the heart relaxes. Upon auscultation, the S1 sound is heard at the beginning of systole, when the ventricles contract and the atrioventricular valves snap shut. On the other hand, the S2 sound is heard at the beginning of diastole, and marks the closing of the aortic and pulmonary valves. Now, if there is a lot of blood coming in during diastole, the S3 sound can be heard right after S2. Finally, the S4 sound can be heard right before S1 in case of stiff ventricles that vibrate when the blood flows in, like in ventricular hypertrophy. If you want to learn more about the physiology of the cardiac cycle, check out this amazing graph!
Source: https://www.instagram.com/p/CaIFKupuAfc/?igshid=YmMyMTA2M2Y=Thank you so much for this pic. Was searching for it for a long time.