What is the best description of the closing of the aortic and pulmonary valves?

The closing of the aortic and pulmonary valves is best described as the 2nd heart sound, also known as "S2". This sound occurs during the cardiac cycle when the ventricles have finished contracting and the pressure within them decreases, leading to the closure of the semilunar valves (aortic and pulmonary valves). The production of this sound signifies the end of systole and the beginning of diastole, allowing the heart to refill with blood.

In contrast, the 1st heart sound, or "S1", is produced by the closure of the atrioventricular valves (mitral and tricuspid valves) at the beginning of ventricular systole. The 3rd heart sound, "S3", is associated with rapid ventricular filling and is more commonly heard in younger dogs or certain pathological conditions. The 4th heart sound, "S4", occurs just before S1 and is related to atrial contraction against a stiff ventricle.

Thus, recognizing the distinct sounds produced during various phases of the cardiac cycle helps in understanding the mechanics of heart function, with the closing of the aortic and pulmonary valves distinctly represented by the 2nd heart sound.