- When you exercise, your muscles need more energy (they respire more). Therefore in order to get O2 to your muscles and CO2 away quicker, heart race increases.
- This increases the rate of blood flow, increasing the amount of oxygen and carbon dioxide going in and out of your muscle cells
- The brain also sends signals to the heart which causes more forceful and frequent contractions
- Released by the adrenal glands, when an organism is threatened, adrenalin binds to the receptors of the heart and causes the cardiac muscles to pump/contract more frequently (increases the amount of O2 supplied in tissues)