Epinephrine

Epinephrine (also referred to as adrenaline) is a naturally occurring hormone produced by the medulla (inside) of the adrenal gland (located above the kidney). The secretion of epinephrine by the adrenal is part of the fight-or-flight reaction, signaling the heart to pump harder, increasing blood pressure, opening airways in the lungs, narrowing blood vessels in the skin and intestine to increase blood flow to major muscle groups, and performing other functions to enable the body to fight or run when encountering a perceived threat.

Another important thing to remember about epinephrine and the body's stress response is that it can be triggered in response to negative stress as well as excitement.