Why do people's hearts beat really fast when they are nervous / excited?

Does it?: Have you checked your heart rate when nervous? Many people who experience anxiety have the perception their heart is beating fast, but when checked the pulse it is not increased. Some times the pulse is increased and this is related to release of adrenaline in the so called "fight or flight" response. Your mind/body is responding to a threat, your muscles and brain need more o2 so your hr^.