Byron’s car skidded across the rain-slicked road, into oncoming traffic. He was able to regain control and avoid an accident. As he sat in his car on the side of the road his blood pressure decreased and his heartbeat slowed. Byron’s blood pressure and heart rate returned to normal levels due to activity within his:

Parasympathetic nervous system

Explanation:

Parasympathetic nervous system (PNS) along with sympathetic nervous system (SNS) makes autonomic nervous system (ANS). ANS controls the functioning of internal processes like digestion rate, heart rate, respiratory rate etc. During a threat or stressful situation SNS is activated. It increases heart rate, respiratory rate and directs blood flow towards peripheral muscles as a part of "fight or flight response". When the situation becomes normal, PNS is activated which restores all the vitals as a part of "rest and digest" mechanism.

Since here Byron almost got into an accident, he was scared a lot due to which his SNS was activated. Eventually when he realized that he is out of danger, his PNS got activated which returned his heart rate and blood pressure to normal levels.

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