How does the body increase heart rate in response to exercise?

Heart rate is controlled by the AUTONOMIC nervous system. This is the branch of the nervous system which does NOT require voluntary thought and action, unlike the SOMATIC branch. This means it relies upon detecting chemical and pressure changes within the bodies blood vessels to make adjustments to the heart rate (without you even knowing). When you exercise, this is essential in order to increase OXYGEN delivery to the body’s tissues as they will be RESPIRING more. During exercise muscles use more O2 and produce CO2 as a waste product (aerobic respiration). This CO2 is carried in the blood to the lungs so that it can be exhaled, but whilst in the blood stream it undergoes a chemical reaction with H20 to form H2C03 (Carbonic Acid). Acidic substances LOWER the pH of the blood (the body controls pH HOMEOSTASIS within very tight ranges to ensure that cells are able to function). This drop in pH is detected by specialised chemoreceptor cells located in the walls of the carotid artery (at the point where the common carotid bifurcates in the neck). These cells send this information to the Cardiac Acceleratory Centre in the Medulla Oblongata in the Brain Stem. In response, The CAC in the Medulla increases the FREQUENCY of impulses along the SYMPATHETIC chain of the autonomic nervous system which terminate on the Sino-Atrial Node in the heart. This causes the SAN to increase its rate of fire, increasing the heart rate. The increase in heart rate allows more CO2 to be removed from the respiring tissues and exhaled by the lungs, causing the pH to return to normal.