By working do you mean showing blood (or fake blood) flowing through tubes? Do you mean for the heart or for the whole body? Museum fabricators have made many such models using clear flexible plastic tubing such as used in aquariums for pumps. In this case a hand pump might work better than an electric one. You could draw/paint your model body or heart on a piece of plywood or corrugated plastic and then fix the plastic tube in place by following a 2-D model from a book or Internet site. Good luck!