Special heart cells create electrical signals that cause the heart's upper and lower chambers to beat in the proper sequence to pump blood through the body. (sciencedaily.com)
You also need to unlock most of the stages by collecting special heart items. (n64today.com)
There is no character levelling - up, with Mario's energy being increased in a similar fashion to in Nintendo's The Legend of Zelda line, with special hearts located in various places that upon collection add an extra section to his health meter. (cubed3.com)