Adrenaline is the hormone responsible and for a very good reason.
Adrenaline is released by your adrenal glands when you sense danger, to prepare the body for ' fight or flight'. Within microseconds of your senses telling your brain of that threat, it has sent the message to your adrenals to increase their output of adrenaline, which is rapidly released into the blood to reach the organs on which it will act. It dilates blood vessels of muscles, giving them more oxygen and sugar, which lets them work harder to help the body escape the threat. It dilates the tiny coronary blood vessels, allowing a greater blood flow to the heart, which can then pump an even greater flow of blood to the muscles and also to the brain to improve clear thinking. The heart also beats faster to further improve blood supply to muscles and brain.