John begins to exercise and his heart rate increases - explain how and why his heart rate increases

His heart rate increases as his cellular demand for oxygen increases and the amount of carbon dioxide in his blood increases, this is due to an increase in the rate respiration because of increased ATP demands. This causes his chemoreceptors, located on his medulla oblongata, aortic sinuses and carotid sinuses to detect a decrease in oxygen and the increase in carbon dioxide. This activates his sympathetic nervous system which then triggers the secretion of the neurotransmitter noradrenaline onto the sinoartial node of his heart. This increases the rate of action potentials produced by the sinoartial node therefore increasing the number of cardiac contractions per minute so increasing John's heart rate.