[robg]

The automatic feedback loop between heart and lungs is really cool. The more you slow your breathing the more you slow your heart. You are choosing to breathe slower via your vagus nerve. The heart responds accordingly.

By contrast the adrenal cortex, a little brain on top of the kidneys, automatically triggers fight or flight with threat detection. By the time you realize it your heart rate is already increasing. Breathing slowly, preferably through your nose, counteracts that physiological stress.

See also the last 100 years of research on the autonomic nervous system.