When you are not expecting something, such as being startled by someone, something, or a loud noise, your nerves become alert. The reason your heart rate increases when you are startled are for a couple of reasons.
One of the reasons is, when your body is moving at a normal rate (it usually moves at mph), your body is at rest. So, if someone were to sneak up behind you and shout something, it sends a signal to your brain and your heart, one of the signals being “Fear!”. I like to think of it as the body waking up the system.
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