What are innocent heart murmurs?

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The normal heart beat produces two heart sounds, named first and second heart sounds. First heart sound is produced when the lower chambers of the heart contract. At this time, the valves between the upper and lower chambers close, producing the first heart sound. Second heart sound is produced when the lower chambers start relaxing after a contraction. They are produced by closure of the valves between the lower chambers and the great vessels arising from them.
Occasionally, a third heart sound may be heard in normal children. A fourth heart sound may be heard in elderly sometimes. Otherwise the presence of third and fourth heart sounds are taken as abnormal. In addition to heart sounds, additional longer vibrations may be heard, which are called as heart murmurs. A heart murmur without heart disease is known as an innocent heart murmur. They are most often heard in children, due to the higher velocity of blood flow.