You have two atria and two ventricles.
At least, I hope so. If you are human.
The two atria beat together and the two ventricles beat together. With each atrial beat, the blood is squeezed from the right atrium through the tricuspid valve to the right venticle and from the left atrium through the mitral valve to the left ventricle.
With each ventricular beat, the blood is squeezed from the right ventricle through the pulmonary valve into the lungs and the blood is squeezed from the left ventricle through the aortic valve into the body and brain. Circulation through arteries, through organs, down to the smallest capillaries and then the blood returns. From the body to the right atrium, from the lungs to the left atrium, ready for the next beat.
The valves are supposed to open in sequence when the atria pump and when the ventricles pump. A valve that doesn't open right is a stenotic valve. A valve that leaks when it is supposed to be closed is a prolapsed valve. The lungs are a low pressure system. If the valve from the left atrium to the left ventricle leaks, there is back pressure into the lungs, because the left ventricular pressure is higher. High pressure in the lung arteries is pulmonary hypertension and difficult to treat. Pressure in the body and brain arteries is systolic hypertension or diastolic hypertension. Either one can damage the left ventricle if it pumps against high pressure for hours, days, years. Have your blood pressure checked at least every two years before age fifty and yearly after age 50.
Sometimes the two atria don't beat at quite the same time. This is a split S1. When the ventricles don't beat at the same time, it's a split S2.
The heart sounds change with a stenotic valve or a prolapsed valve. With atrial stenosis there is a harsh crescendo murmur after the S2. With mitral valve prolapse there is a decrescendo softer murmur at the same time. Those are the two most common murmurs. Prolapse can cause slow damage over time, especially between the atria and ventricles. Aortic stenosis is monitored: when the valve area is reduced too much, the valve must be replaced. Otherwise it can cause sudden death.
Your heart beats in 2s and 2s.
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