Why does your heart rate increase during exercise?

When you exercise, your muscles need more oxygen and nutrients to produce energy, and they generate more carbon dioxide and waste products that need to be removed. Your cardiovascular system responds by increasing heart rate to pump more blood to working muscles faster. This is triggered by the nervous system, specifically the sympathetic nervous system releasing adrenaline and noradrenaline, which directly stimulate the heart to beat faster and harder. Additionally, chemoreceptors in your blood vessels detect rising carbon dioxide levels and falling oxygen levels, sending signals to the brain's cardiovascular control center, which then amplifies the heart rate response. The result is increased cardiac output, ensuring muscles receive the oxygen they need to sustain physical activity.