As has been said, the lungs are bigger, but deer tend to crouch a bit when they hear the bow.
So, I've subscribed to the school of thought that says to aim at the heart/lung border.
Either hit will kill, but I like theidea of the heart pumping out blood faster with a double lung hit. That makes a lot of sense. Faster loss of blood = faster loss of consciousness.

Remember that bones are an archer's worst enemy, and while you can see the outline of the shoulder through the fur, the spine has to be visualized by just knowing where it is. The spine takes up the top third of the deer's chest, and is not nearly as high as many archers believe. If you hear someone talking about putting an arrow through "the void" in the top of a deer's chest, they simply don't fully understand that they can shoot over the spine, and still hit the deer. Check out the pics below. These are the best I have seen.

Since arrows do notdeliver the blunt trama and shock of a bullet or slug, rapid blood loss is the way they do their job. You are wise to investigate the difference in shot placement before flinging your first arrow at a deer. I salute you for being responsible.

-Bulz

I get the Men's Channel and on one of the shows on there, they advertise a DVD series about shot placement for different angles and such. I can't remember the name of the show that advertises the video, but maybe someone can. This may be something you could check into as well to get an idea of where to shoot in what situation.

So your saying that I should aim inbetween the heart and the lungs? I had thought of that, but I didn't know how much space was between the two organs, and I would hate not to hit either of them! As for being responsible, I wish my parents could realize that! I'm 17. You have been a lot of help.

Very well said and done

This is my first time hunting with a tree climber, first time with a crossbow generally speaking how high should i be? this is my second time deer hunting at 43 years young.

Last year I had a terrible shot on a small 11 point. Hit him in that soft valley behind the gut and in front of the hind quarter. He ran five yards and fell over. Never would I aim there, but must have hit something major.

So might mite, have you tried it again? I wonder what was back there. It may be the femoral artery. I know I probably didn't spell that right. But I think I'll stick towards the front of the deer.