Yasmeen and her girlfriend recently got into a major fight, and there was a lot of yelling and screaming before yasmeen stormed out of the house and drove away. a few minutes later, she is calming down due to increased activity in her nervous system.

She is calming down due to increased activity in her "parasympathetic" nervous system.

The parasympathetic nervous system works in relation to the sympathetic nervous system, setting off the body to emit hormones to diminish pulse and heartbeat, initiating a relaxing reaction. Breathing heavily and carefully fortifies the parasympathetic nervous system to initiate this reaction.  

i honestly have no idea