Cardiac Limits

In shrews and hummingbirds, small hearts supply energy to fast moving muscles that work almost continuously. Because these animals consume so much energy, to be awake is to eat. Their hearts beat so fast because small pumps can eject only a small stroke volume. If the heart rate is too fast, however, diastolic filling time may shorten, so much that the ventricles cannot fill and the coronary vessels cannot supply the heart muscle. If a heart spends too much time contracting in systole and cannot compensate in diastole, it can fail. Insects avoid this uniquely vertebrate problem of high output heart failure with an open circulation.

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