Systolic murmurs occur during systole (between the first heart sound (S1) and the second heart sound (S2). Systolic murmurs are further classified by timing within systole: mid or late. Holosystolic murmurs are systolic murmurs lasting for all or nearly all of systole.
Mid-systolic ejection murmurs are often heard with a rising then falling intensity. This is called a crescendo-decrescendo murmur. The murmur sound is due to blood flowing through the semilunar valves.