Living Environment Regents Practice Test 2025 – Complete Exam Prep

The increase in heart rate during exercise is primarily due to the rise in carbon dioxide levels in the bloodstream, which is a natural response of the body to physical activity. When a person exercises, the muscles require more energy, leading to an increased rate of cellular respiration. This process generates not only energy but also produces carbon dioxide as a byproduct.

As the level of carbon dioxide rises, it stimulates the respiratory center in the brain, which signals the heart to beat faster. This increased heart rate helps to pump more blood, and therefore more oxygen, to the active muscles while also assisting in the removal of carbon dioxide. This physiological response ensures that the body meets the heightened demand for oxygen and effectively manages waste products during exercise.

In summary, the correct answer highlights the role of increased carbon dioxide levels in triggering a faster heartbeat to support the body's needs during physical activity.