Assuming an anatomical figure from a popular anatomy book is drawn completely from scratch, and I have the PSD file to prove it, would it still be considered copyright infringement, even though it's of the human body? I mean the muscles, arteries etc, are the same for almost everyone.
Yes, if you use one image as basis then you are copying. And copying violates copyright. Copyright does not actually care about the method of copying. As you yourself admit to copying it. The way you circumvent this is by using several different references.
Now, copyright works slightly differently in different parts of the globe. So some juristictins might not consider this copying unless its really a carbon copy. On the otherhand same juristiction may not have fair use provisions so they are using stricter copyright definitions instead.