Actually, if you look carefully at the text, Shylock is very careful to say that the pound of flesh could be cut from wherever he chose. Therefore, in theory, he could have chosen to cut out the heart of Antonio. If we look back at Act I scene 3 which is an important scene for many reasons, we see the exact terms of the "merry bond" that Shylock draws up with Antonio:
...let the forfeit
Be nominated for an equal pound
Of your fair flesh, to be cut off and taken
In what part of your body pleaseth me.
So therefore, whilst Shylock does not specify which part of the body he will take as forfeit, he could have, in theory, chosen to cut out the heart of Antonio as his pound of flesh. I suppose we need to think about Shylock's motivation in answering this question. He is after revenge for the wrongs that Antonio has committed against him and his race, and thus his desire is to cause pain, to mutilate, and perhaps to kill.