Why do we feel calmer after a deep breath?

You're nervous before a big test, or angry after an argument, or worried about something you can't quite name. Someone says, "Take a deep breath." You roll your eyes — how could gulping air possibly help? — but you do it anyway. And somehow, impossibly, you feel a little better. What just happened?
Inside your chest, your heart has been hammering away, pumping fast because your brain decided there's danger. Not tiger-chasing-you danger, but the modern kind: embarrassment danger, failure danger, too-much-happening-at-once danger. Your brain can't tell the difference, so it hits the same alarm button our ancestors used when they actually ran from tigers.
