What exactly are physiological reactions?

Imagine you are a young teenage girl. You are waiting in the math hall, and that handsome senior with an air of indifferent confidence strolls past you on the way to calculus. Normally you are watching unseen, but today he looks you in the eye and says “Hi, how’s your day going?” Your heartbeat increases, you breath faster, you start to feel warm, and your face begins to turn red – otherwise known as blushing  – these are the physiological effects of surprise, even a pleasant surprise.

Now, imagine you are a soldier sent to scope out the number of men guarding an enemy compound. After stealthily scaling the chain link fence, you slide between the fence and building, treading softly, trying to conform to the shadows.  You can hear unintelligible conversation. Then, there is sudden silence followed by the click of a semi-automatic weapon…