Adrenalin is a hormone secreted by the adrenal glands in case of any danger or emergency or any kind of stress. It is secreted directly into the blood and is transported to different parts of the body.
When secreted in large amounts, it speeds up the heartbeat and hence supplies more oxygen to the muscles. The breathing rate also increases due to the contractions of the diaphragm and rib muscles. It also increases blood pressure. All these responses enable the body to deal with any stress or emergency.