Turning pale

In a fight or flight situation the skin turns pale because the blood vessels to the skin contract and the blood flow goes rather to the muscles.

This evolutionary biological reaction prevents the skin from bleeding and wasting the so much needed blood in a fight or flight situation.

Butterflies in the stomach

Adrenaline stops temporarily digestion, which is why you can feel nauseous. Blood leaves the stomach and goes where it is required.

Our ancestors needed all the blood they could in their legs and skeletal muscles to run from that prehistoric saber-toothed tiger.

So in a fight situation, your body decides that dodging a powerhouse punch coming to the chin has more priority than digesting that burrito you ate in lunch.

Sweat

Your body can’t afford being overheated in a survival situation, so sweat glands open up to cool down the body.

Knees going jelly

This trembling is a direct response to our fight or flight system. The sympathetic nervous system (Which activates/deactivates the fight or flight mode) releases adrenaline to the skeletal muscles which turns your body into a McLaren P1.

Now your body is in full mode to throw down. In case of no action, the adrenaline released in your blood stays circulating in muscles and causes your limbs to tremble.

So once you throw down and action takes place you will be well served with that adrenaline in your blood and you will not feel the trembling anymore.

Ask any of your friends, you will have the same answer that once you start action the jitters go away with all its symptoms.