Second messenger systems are an indirect way cells use to guide responses within the cell to certain stimuli, often from outside the cell. There are receptors on the cell membrane which receive the stimulus from the exterior of the cell. The receptor cell, in turn, coordinates the appropriate response from substances or organelles within the interior of the cell. This indirect approach works on a scale similar to the organs in the endocrine system, which create chemicals that are designed to elicit a response from a target tissue somewhere else in the body. The response produced by the adrenal glands secreting epinephrine is a good example, for both the endocrine system and the second messenger system. Epinephrine, commonly known as adrenaline, is the hormone that produces the "fight or flight" syndrome, producing dilated pupils, elevated heart rate, and heightened blood sugar levels.