We turn white when we are frightened because the blood in our cheeks is diverted to do a more urgent job. At the same time our hearts begin to beat much faster, and we breathe more quickly.
When we sit still, our hearts beat at about 70 to 80 times a minute, pumping the blood through our bodies. The blood carries nourishment from food, and oxygen from the air we breathe, both of which are vitally necessary for the body to function.
If we take violent exercise, our muscles need to work much harder and faster than when we are sitting or walking. They therefore need extra nourishment and oxygen. The nerves carry the message to that part of the brain called the hypothalamus, the center of an automatic nervous system in control of internal bodily functions such as the pumping of the heart, breathing and digestion.
Impulses from the hypothalamus travel down the spinal cord and excite other nerve cells - "sympathetic" neurons - which end in the center or medulla of the adrenal glands just above the kidneys. These glands release the hormone adrenalin into the bloodstream which causes the heart to beat faster and more efficiently, dilates air passages in the lungs and the blood vessels that supply the muscles, and increases the concentration of energy-giving glucose in the blood.
In fact, when we are frightened, exactly the same physical changes take place and our bodies are immediately and efficiently prepared for the violent exercise of flight or fight without any voluntary effort on our part.