Through the interactions between the sympathetic and parasympathetic nervous system.

Here’s how it works:

-When you inhale (whether through the mouth or the nose), the diaphragm moves down because the lunges expand. Your heart gets a little bit bigger in that expanded space. As a consequence, whatever blood is in there is now moving slower in that larger volume. There’s a little group of neurons, the Sinoatrial Node in the heart, that pays attention to the blood flow in the heart. It will send a signal up to the brain that blood is moving more slowly through the heart. The brain then sends a signal back to the heart to speed the heart up.

If you want your heart rate to go faster, inhale longer, more vigorously than your exhales.

If you want to slow your heart rate down, what you want to do is exhale. When you exhale, the diaphragm moves up, the heart gets a little bit smaller. Bood flows faster through a smaller pipe. The Sinoatrial node sends a signal to the brain to slow the heart down.

If you want to calm down, you want to make your exhales longer or vigorous than your inhales.

Master Stress, Huberman Lab, 27mins.