To become a dragon one needed a dragon's heart.
For lesser draconic species like wyverns and aerodactyls, their existing hearts were close enough to that of a dragon, hence they only got an upgrade of the organ, but for non- draconic specimens like Max, they needed to be transplanted one.
While Max's original heart could only pump blood, the dragon heart would be able to pump both blood and mana through his body with both the blood circulatory system and the mana circuits connected to the heart.
In the dragon world, it was general knowledge that the older the dragon the stronger it was unless it had begun regressing and this was because of the special characteristic of the dragon heart.
