Q How does your brain tell your heart to constantly beat?
— Jane Lowy, Madison, Wis.
A Lee Eckhardt, cardiologist, University of Wisconsin-Madison School of Medicine and Public Health:
In the course of the day, your heart will beat somewhere around 100,000 times, and over a calendar year it might beat up to 35 million times. Over the course of a lifetime then, your brain and your heart have to work together to engineer 3 billion heartbeats.
The brain gets to take a little break here because the heart will actually beat all by itself. There’s something in the heart called automaticity. That means that the heart, even if it’s disconnected from the brain, will continue to beat at a set rate — something called the intrinsic heart rate.
For different people, that intrinsic heart rate can be slightly different, but it usually sits somewhere around 90 to 110 beats per minute.
If you have a heart transplant and the heart is then transplanted into another human, it’s not connected to the brain, but that heart continues to beat at a set intrinsic rate.
In addition to the intrinsic heartbeat that the heart has all by itself, the autonomic nervous system is a separate part of the brain and the brain function that can either speed up or slow down your heart.
When you’re resting or sleeping and you don’t need a very fast heart rate, then the parasympathetic nervous system slows down your heart via a nerve called the vagus nerve. On the other side, if you start exercising or you go from a chair to run up a flight of stairs, then you need to have a much faster heart rate when you go from rest to exercise. That is also controlled through a second set of nerves through the sympathetic nervous system.
With that regulation, the heart rate then has a great deal of variability throughout the day, depending on what your body needs.