I noticed a pretty nice model of a heart on www.3Dcafe.com. You question is pretty general, so not sure how to answer. I would probably sculpt it with NURBS. Then for objects like the aorta, I could trim a hole, and put a branch above it and then use Blend surface to join them. Metaballs as already mentioned might do well for the chambers.
If you have a surface model, you can use surface deformation to slide viens and arteries over the outside.
What ever you do, work the shape out on paper first, so you don't have to rework constantly.
Urp