The cardiac cycle is one full heartbeat: the ventricles fill, contract, eject blood and relax. Pressure changes in each chamber open and close the valves at exact moments.
Diastole
The ventricles relax and the mitral and tricuspid valves open, so blood flows in from the atria. At the end, the atria contract and add the last part of the filling.
Systole
The ventricles contract, pressure rises and the mitral and tricuspid valves snap shut. When ventricular pressure passes the pressure in the aorta and pulmonary artery, the aortic and pulmonic valves open and blood is ejected.
S1 and S2
S1 is the closing of the mitral and tricuspid valves at the start of systole, loudest at the apex. S2 is the closing of the aortic and pulmonic valves at the end of systole, loudest at the base; it normally splits on inspiration.
S3 and S4
S3 comes in early diastole when blood rushes into a large, overfilled ventricle; it can be normal in children and young athletes but suggests heart failure in older adults. S4 comes just before S1 when the atria push blood into a stiff ventricle, as in long-standing hypertension.
At the bedside
Feel the carotid pulse while you listen: S1 comes just before the pulse. Murmurs between S1 and S2 are systolic; murmurs after S2 are diastolic.
Related guides
Written by Making Synapses. Topic reference: OpenStax, Anatomy and Physiology 2e (no text adapted).