You know, if you aim for the heart, youre still aiming at the lungs ,since the heart lies in between the lungs.
The problem w/ aiming for the center of the lungs is, you will often hit high when the deer drops, as they often do, when the bow goes off. High hits are bad ,since they often leave much less, or even no blood on the ground.
what I do is divide the deer in 3 equal parts long ways,aim for the bottom of the middle section.