What system would be activated if you were surprised?

The sympathetic nervous system (SNS)

The sympathetic nervous system (SNS) is a division of the autonomic nervous system (ANS) that is responsible for the body's "fight or flight" response. When you are surprised, your SNS is activated and your body goes into a state of high alert. This can cause your heart rate and breathing to increase, your blood pressure to rise, and your muscles to tense up. The SNS also helps to release stress hormones, such as adrenaline and cortisol, which can help you to stay alert and focused.