If you live to 80, your heart will beat an average of three billion times. That’s right: billion with a b. Faster when you exercise, slower when you’re at rest, it does its job 24 hours a day, seven days a week. What’s even more amazing is how your heart rate is controlled by the nervous system. I’ve always found it fascinating and here’s why.

The pacemaker of the heart we talked about on Thursday gets signals from nerves that tell it . . .

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