What is the mechanism which leads to an increase in heart rate during exercise?

Exercise leads to an increase of CO2 in the blood. This lowers pH as the CO2 binds with water to form carbonic acid. This pH change is measured by chemo-receptors in the carotid arteries. This change then signals to the hypothalamus which then initiates an electrical impulse down the sympathetic pathway to the heart to increase SAN node triggering, leading to an increase in heart contractions.