(Science: hormone) a hormone produced by the adrenal medulla in mammals. It can be produced synthetically for medical purposes. It is secreted by the adrenal medulla in response to low blood glucose, exercise and stress and causes a breakdown of glycogen to glucose in the liver, encourages the release of fatty acids from adipose tissue, causes vasodilation of the small arteries within muscle and increases cardiac output. This is a hormone secreted by an animal in 'fight or flight circumstances, which basically means when the organism has to exert higher than average thinking and movement in a particular situation, usually in danger. Adrenaline increases heart rate, and in general makes the organism more alert to their external environment.