During exercises, our muscles needed energy for contractions. The heart pumps more rapidly and more powerfully during this time for a few reasons.
First, more oxygen and nutrients like glucose can be transported to the heart. Muscle cells uses oxygen and glucose for aerobic respiration to generate energy.
Secondly, more carbon dioxide and waste can be transported away from the muscles. Waste products from aerobic respiration includes carbon dioxide or water etc. The faster the heart pumps, the faster the blood gets transported so more waste can be carried away.
Thirdly, lactic acid can be carried away too. When oxygen supply is not enough, the cells undergo anaerobic respiration to generate energy. This type of respiration does not require oxygen, but a waste product called lactic acid is produced. Lactic acid must be carried away from cells because they're harmful. Therefore, the faster the heart pumps, the more lactic acid gets carried away.
Last but not least, heat can be carried away too. Part of the energy generated is turned into heat. Therefore, in order to help us cool down, blood travels fast to carry heat away from the heat generating muscle cells.