When you encounter a stressful, exciting, dangerous or threatening situation, the adrenal glands and some central nervous system neurons quickly release adrenaline (epinephrine) into the blood stream. This triggers the body's "fight or flight" response, which gives you the ability to quickly escape a predator or respond to a threat.

Shortly after adrenaline was discovered in 1901, it was found to have medical uses as well. Today, it's artificially synthesized and used to treat myriad medical conditions. For example, epinephrine is the active ingredient in the EpiPen, the autoinjector used in emergencies to treat serious allergic reactions or anaphylaxis. Epinephrine is also the primary drug used to reverse cardiac arrest. And it's sometimes administered to patients during acute asthma attacks or applied to wounds to slow bleeding.

https://science.howstuffworks.com/adrenochrome.htm