An easy arithmetical calculation indicates that the human heart must beat nearly 50,000,000 times in the course of a single year; and this cardiac performance continues undisturbed for three score years and ten in many a person. Even from the standpoint of the mechanism involved, this is a phenomenon that challenges attention. The layman is astounded by the apparent ability of a small muscular organ to work both without interruption and without fatigue; but the student of physiology soon learns to interpret the events of the cardiac cycle in other ways. He can readily demonstrate for himself that the life of the heart is by no means all work. Each contractile effort is followed by a period of rest and recuperation, without which the seemingly continuous performance would become impossible.
The average duration of the cardiac cycle occupied by systole and diastole of the heart in adult man is about
DURATION OF THE SYSTOLE OF THE LEFT VENTRICLE. JAMA. 1926;87(10):758. doi:10.1001/jama.1926.02680100042012
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