The human body is amazing. Those of you know know me know I am in love with anatomy and physiology. I find it hard to understand people who dont look at the processes of the human body with awe.
If you are going through extreme physical stress your body secretes adrenaline which blocks out the pain from physical trauma. It slows non-essential function like digestion and diverts oxygen rich blood to the muscles. Your pupils dilate and contract much more quickly, signals are transmitted too and from the brain at an increasing rate. Everything about this chemical is designed to pull the body out of danger.
Even a simple non-life threatening incident like stepping on a pin invokes a reaction called a reflex-arc. This means your body knows there is a threat, it pulls your foot away from the pin before the pain message is even sent to the brain and interpreted. Its a purely localised self-defense mechanism that can occur at any point of the body.
If you are exposed to severe psychological trauma your mind takes you where you need to go in order to survive it. And then, when you are safe it blocks the memories in order to prevent them having a detrimental effect.
Both the body and mind are full of failsafes designed and tested over the last billion or so years in order to keep you alive.