The answer is: "fight or flight"
During the fight or flight scenario, our mind entered a mode where it evaluate the source of threat that we face , analyze it , and create a conclusion whether fighting it or run away from the situation would be the best possible option.
In the case above, the snake is what considered to be the source of threat. The increase in heart rate is caused by a spike in adrenaline that help us focus. The fact that she is run away means that her mind decided that running is the best course of action during her fight or flight process.
