The phrase 'rush of adrenaline' is common in action movies and stuntmen. But what is a adrenaline really? Adrenaline is the hormone and neurotransmitter also known as epinephrine. Hormones are used to quickly pass messages to organs in the body en masse, it is controlled by the endocrine system. Epinephrine in particular is a major factor in the 'fight or flight' response. It's response is to increase blood flow to muscles, output of heart, pupil dilation, and blood sugar. All symptoms of the body response to fear. It does this by effecting the alpha and beta receptors.
Alpha receptors have several common effects such as vasoconstriction of veins and decreasing motility of smooth muscle in intestines. Specific vessels that are targeted in fear are the skin, gastrointestinal system, kidney's and brain. They also are the reason behind contractions in pregnancy. Beta receptors are more commonly used in relaxing smooth tissue rather then heightened tension in alpha receptors, but they also increase cardiac output (heart rate, conduction velocity, stroke volume).
As epinephrine controls a great deal of the body when forcing it into a high activity state it can also be used to a beneficial medical way. It can be used to treat cardiac arrest with its control over beta receptors, asthma - alpha, croup - alpha. Side effects due include a state of anxiety as the body is forced into and artificial 'fear state', but the benefits outweigh the costs.
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