The four cardiac valves are classified into two types - the atrioventricular [mitral and tricuspid] and the semilunar [aortic and pulmonic] valves. This is an important distinction. During systole, atrioventricular valves are closed and semilunar valves are open.
In diastole the opposite is true. Atrioventricular valves are open to allow ventricular filling and semilunar valves are closed to prevent backflow of blood to the heart from either systemic circulation or pulmonary circulation.
Because atrioventricular and semilunar
valves function oppositely in the cardiac cycle the effects of stenosis
and insufficiency are different. Stenosis of the semilunar valves causes
systolic murmurs (AS,
of the atrioventricular valves causes diastolic murmurs. Insufficiency
of the semilunar valves is associated with diastolic murmurs (AI,
of the atrioventricular valves causes systolic murmurs (MR,