What does exercise do for the heart to keep it healthy?

Improves strength. Your heart is a muscle too. Regular exercise strengthens the muscle so that it pumps more efficiently. Just part of a healthy lifestyle to help achieve a long life of high quality.
Many changes. Exercise helps not only the heart but the entire body! you get enlargement of the heart like any muscle that you use regularly and increased blood supply. New receptors are created that help activate muscles and use oxygen more efficiently. There, are many other changes that are positive too. Good luck.