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Text В Nature #f the Heart Beat

The fact that the heart, completely removed from the body, will go on to beat for a time shows that its beat is «automatic», i.e., does not require nerve impulses.

The beat is rhythmic: it is not jerky; the ventricles relax fully before the next contraction. This is explained by a special property of cardiac muscle tissue. The period of time during which the muscle is not re­sponsive to a stimulus is called the refractory period. It is characteristic of the heart muscle to have a long refractory period. When the heart muscle is stimulated, it will contract but will not respond again to that stimulus (though it may respond to a stronger one) until it has relaxed. This rest period is occupied by the heart filling with blood, in prepara­tion for the next beat. Even the heart forced to beat rapidly maintains a perfectly rhythmic beat; although the beats come closer together, there is always that little rest period in between.

The heart is a pump, but a double pump; the volume expelled by the light ventricle is the same as that expelled by the left. When exercise is suddenly undertaken, the «venous return», i.e. the blood returned to the heart through the veins, is suddenly increased. For a few beats the right ventricle does put out more blood than the left, but soon the additional blood has passed through the lungs and is entering the left ventricle. lYom then on, both put out the same amount.

108 ♦ Learning to Understand a Medical Text

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