It was his heart, they gave him the super soldier serum when he protected everyone from the fake grenade throw. They knew he was going to be the protector.

Steve Rogers is the man of truth. He is can do anything for there nation & there peoples. When this qualities are seen in Steve so he has been given the super Soldier serum which turned him into Captain America the brave soldier.

He was always like a Selfsacrificing leader the powers have only made him physically stronger