Under ordinary conditions the heart beats are controlled by certain nerve cells which are located within the heart itself, and these cause the heart to beat even while the brain is asleep. This explains why the heart beats when the brain is asleep, and the fact that the brain when asleep does not exercise its functions, shows how necessary this arrangement and the control of ordinary heart beats is. If this were not so, we should not be able to live while asleep. It is just like the management of a great business in this sense. The general manager of a great business has control of the entire works, but there are occasions when he must be thinking of only one thing in connection with the business, and so he must have his organization so complete, that the parts which he cannot be thinking about at the time will do their work just the same. So he surrounds himself with competent assistants, who look after certain departments while he is busy or away or asleep, and if anything goes wrong while he is away, he calls on special forces to set things right. Now, the brain is the general manager of the whole body and has these nerve cells in the heart as a sort of assistant manager to look after the heart beats in ordinary conditions, and to keep the heart going while he is asleep. But, by reason of his office as general manager, the brain has a special way of sending orders to the heart through special nerves which run from the brain down each side of the neck to the heart. There are two pairs of these special nerves. One pair, if set in motion, will make the heart beat faster, and the other pair will make the heart beat more slowly. Why Do Our Hearts Beat Faster When We Are Running? When you start running, the brain knows at once that your legs and other parts of the body will need more blood to keep them going, and so the brain sends down orders through his special nerves which make the heart beat faster, to get busy, and they do. Then when you stop running, your heart is beating faster than necessary--there is really an oversupply of blood being pumped through your system for the time being, and that makes you uncomfortable, until the brain sends word through the other set of nerves to the heart to slow down the heart beat. It is better to stop running gradually, to give the heart a chance to get back to its normal beat gradually also. Why Do I Get Out of Breath When Running?