Understand that the heart rate changes during exercise and under the influence of adrenaline.
Excercising involves muscle contractions and this muscle contract requires energy, this energy is produced by increased levels of muscle cell respiration. So the muscle cell needs more oxygen and the removal of carbon dioxide. By increasing heart rate we can supply more blood and therefore more oxygen and we can remove more carbon dioxide.
By increasing the output of the heart, more blood is sent to the heart and more carbon dioxide is moved to be excreted by the lungs. The heart can also be influenced by hormones and the particular hormone is adrenaline; this is produced in the adrenal glands which sits on top of the kidney.
This travels to the heart through the blood stream and increases in heart rate. The stimulus for the production includes fear, anxiety, danger. This results in an increase in heart rate. A faster beating heart sends more oxygen and glucose to the body so that they can have more movement or exercise