heh, I have a great idea for this: muscle wire. If you want it to pump blood as realistically as possible, I’d build two sets of two (flexible) cylinders with valves in between them and valves at both ends, wrap each cylinder in nitinol muscle wire, and set up a microcontroller to fire them in sequence. The wire contracts when heated, and if fired correctly, the top two cylinders (atria) will pump into the bottom two (ventricles), which will pump out to the lungs/body, which will return it to the atria. You could even hear it pump!
Btw, the muscle wire would represent muscles and the microcontroller the autonomic nervous system.
Btw, if you need to find nitinol, http://www.imagesco.com.