The structure of mitochondria contains the foldings in the inner side called "cristae" which increase the surface area of the mitochondria. The cristae are important to mitochondria as well as cell as cristae embody the ATP synthase enzymes which help in the formation of the ATP molecules.
Heart cells require more energy to pump the blood from the heart to the body so it needs a more mitochondrial number in the cells with more infoldings to synthesize more ATP.
