Science Photo Library / Via Batsford

"[Glands that produce adrenaline] are controlled by the hypothalamus, the part of the brain responsible for instinct and emotion," writes Salter. We always have small amounts of adrenaline in our blood, but when we're stressed we get more.

"It widens the airways of the lungs and constricts small blood vessels. This makes the muscles work harder and produces a ‘fight or flight’ response," he wrote.