A second messenger system is an indirect way of influencing particles and substances inside the cell itself, on the interior of the cell membrane. Substances from outside the cell bind to specific receptor sites on the cell membrane. These receptor sites in turn initiate responses to influence specific particles or organelles within the cell's cytoplasm. A good example of this would be the adrenal glands production of the hormone epinephrine. Epinephrine is also known as adrenaline, and is the hormone responsible for producing the "fight or flight" response in organisms. It is responsible for heightened heart rate, dilated pupils, and elevated blood sugar levels. The adrenaline acts as a second messenger, being produced by the adrenal glands, then being secreted directly into the blood stream to affect specific target tissues within the body.