Your face turns white because blood is diverted to another part of your body. The heart pumps faster and stronger pushing blood to the limbs muscles and adrenaline is released into the bloodstream. In fact, your body is getting itself ready to run or even fight.
When danger is past the body quickly returns to normal. Dangerous situations trigger the fight-or-flight response in the body, the direct result of which is the release of the hormone adrenaline into the bloodstream. The effect of this hormone is increased sweating, dry mouth, pupil dilation, an enhanced sense of smell and pale skin.
Blood flow to the surface of the body is reduced, which is one of the main reasons why your face turns pale during a nervous or scary encounter with someone/something.
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