Adrenaline is a hormone produced by the adrenal gland in the body. When it is produced it stimulates the heart-rate, dilates blood vessels and air passages, and has a number of more minor effects. Adrenaline is naturally produced in high-stress or physically exhilarating situations.

The term "fight or flight" is often used to characterize the circumstances under which adrenaline is released into the body. It is an early evolutionary adaptation to allow better coping with dangerous and unexpected situations. When an animal is threatened, the options are usually either to stand its ground and fight, or run away as fast as possible. Both responses would require extra supplies of blood and oxygen in the muscles. Fright causes the brain to send signals to the renal glands which start pumping large amounts of adrenalin into the bloodstream. This increases the heart and breathing rate in preparation for the ensuing action. With dilated blood vessels and air passages, the body is able to pass more blood to the muscles and get more oxygen into the lungs in a timely manner, increasing physical performance for short bursts of time.

The adrenal glands may be found directly above the kidneys in the human body, and are roughly three inches (7.62 cm) in length. Norepinephrine (or noradrenaline) is also released from the adrenal glands when they are active. In a healthily functioning human, approximately 80% of the released substance is adrenaline, and the other 20% is norepinephrine.

Norepiniphrine (also called noradrenaline)is a molecule made in the brain and limbic system, and involves only a slight modification from the basic adrenaline structure. This molecule, however, serves a much different purpose. It is one of the main neurotransmitters, which means it is the chemical which jumps the gap (synapse) between nerve endings, so transmitting the signal between one nerve and the next. When it is working inside the brain, it gives rise to our thought processes and emotions. Another one of its main functions in the body is to maintain muscle tone in the blood vessels, thereby controlling blood pressure. People who suffer from hypertension (high blood pressure) are often given a drug called reserpine which works by greatly reducing the amount of norepinephrine in the nerve endings.

Adrenaline is the favored treatment for anaphylactic shock, and should be administered immediately if a person begins exhibiting severe allergic reactions. Dosage should be assigned by a licensed medical professional in advance, and instructions should be given on how and where to administer the shot — injection in the wrong place can have serious consequences, including gangrene. In America, while the term adrenaline is still used popularly, the medical community refers to it as epinephrine. This choice was made in response to the trademarking of a similar term (adrenalin) by a pharmaceutical company. The terms are still used interchangeably in most speech however, and if you request an adrenaline shot, any doctor will understand what you are asking for.

Adrenaline was the first hormone to be identified, and was successfully synthesized in 1904. It is part of a family known as biogenic amines, which includes serotonin and histamine, among others. Its specific compound group is the catecholamine group, which also includes norepinephrine and dopamine. Sustained high levels of catecholamines in the blood are a good indicator of chronic stress. It may be important after a particularly stressful situation to 'work off' the adrenaline that has been released into your system. Our ancestors handled this naturally through fighting or other physical exertion, but in the modern world, high-stress situations often arise that that involve little physical activity. This can leave high amounts of adrenaline in the body, resulting in insomnia and jittery nerves.