I think pretty much everything in your body is specialized and is "meant" for something. No matter how hard you try, you're not going to get your heart to replace your pancreas or kidneys. And you can't replace your lungs with brain tissue. Sure, doctors can transplant a toe to replace a thumb or a heart blood vessel with a vein from a leg with great success, but it's doing pretty much the same function, just in a different place.

As a whole, humans are quite adaptable to their environment, but specific body parts tend to be specialized.