Epinephrine

Epinephrine, also known as adrenaline, is a hormone. It's name comes from the Greek phrase epi nephros, meaning "from the kidney", whereas "adrenaline" comes from the Latin phrase ad renal, which means the same thing. It is secreted by the adrenal glands. Epinephrine is released during the fight or flight reaction, causing the body to shift into "overdrive". The pupils dilate, blood vessels constrict, and the immune system weakens, along with other changes.