When I was in college, a nursing professor explained fight or flight so well I've never forgot it.
A bear is after you, what happens in your body? Your pupils dilate so you can see better to get away. Your body floods with adrenaline for strength. Your body sends all your blood to your brain and heart. It will bypass your stomach say, if you have food in your tummy. You don't need to digest food when you are running for your life. You need blood and oxygen, you need adrenaline and to see to get the heck out. Your mind clears of anything except the current situation so you can think. All to get away from the darn bear. It's a short process that last until you get away or fight to live.