During exercise the heart rate increases to increase the supply of oxygen to respiring cells and to remove CO2. Heart muscle is myogenic. The SAN determines the rate at which the heart contracts. But the SAN can be stimulated by the nervous system.The medulla oblongata has two centres in the cardiovascular centre of the brain which can modify the heart rateThe cardiac accelerator centre sends impulses to the SAN via the accelerator nerve which is part of the sympathetic nervous system. This increases the heart rate in response to a decrease in the pH of blood.Chemoreceptors in the carotid artery and aorta are sensitive to pH. Carbon dioxide dissolves in blood to form carbonic acid which lowers the pH of blood. The chemoreceptors increase the frequency of impulses to the medulla oblongata to increase the heart rate.The heart rate increases, more CO2 is removed by the lungs and the pH returns to normal. The chemoreceptors detect the normal pH and reduce the frequency of nerve impulses to the medulla oblongata. The vagal centre then sends impulses along the vagus nerve of the parasympathetic nervous system to the SAN. The rate at which the heart contracts is reduced.