The adrenal glands make the hormone adrenalin when you are frightened, angry or under stress. Adrenalin prepares the body for fight or flight, which means getting ready either to face a danger or to run away from it.
Adrenalin binds to receptors on its target organs and causes several changes. It causes an increase in heart rate, so the heart pumps blood around the body faster. Blood pressure rises. More blood flows to the muscles, so they receive more oxygen and glucose for respiration and can release more energy for movement.
Adrenalin also raises the blood sugar level. It stimulates the liver to change glycogen into glucose, and the glucose is released into the blood. This gives the muscles extra glucose to use.