What physiological event is signified by S1?

S1, also known as the first heart sound, is primarily associated with the closing of the mitral and tricuspid valves. This event occurs at the beginning of ventricular contraction (systole) when the ventricles are filled with blood. As the ventricles contract to pump blood into the aorta and pulmonary artery, the pressure within the ventricles rises above that in the atria, causing the mitral and tricuspid valves to close. The closure of these valves results in the characteristic "lub" sound of S1, which is crucial for the proper functioning of the heart because it signifies the start of the blood being ejected from the heart.

Other choices refer to different phases of the cardiac cycle and involve different valves. The closing of the aortic and pulmonic valves, for instance, occurs during S2, which marks the end of systole and the beginning of diastole. Similarly, the opening of the mitral and tricuspid valves happen during diastole when the ventricles are filling and the heart is relaxed, and the same is true for the aortic and pulmonic valves which open to allow blood to flow into the aorta and pulmonary artery during systole. Understanding these phases is crucial for