I have been asked so many times how I made my human heart cake, so I thought it would be easier to just post the instructions:

http://cakecentral.com/gallery/1323197/human-heart

2 regular shaped heart cakes (8), cooled and filled.
Then carve to look more anatomical; ice with Buttercream.
For the valves, I cut out holes in the cake, ice the holes then airbrush black on that BC.
I then covered the cake with fondant, and then I rolled fondant valves and put them into the holes, and smoothed them to look like they were all one piece.
I put some smashed paper towels into each valve and let it set until the fondant set up hard.
I rolled out fondant worms to make all the veining in 2 different sizes (arteries/veins) and applied to the heart.
Once the valves set up, I airbrushed the whole heart (I did airbrush the insides of the valves black to give it more depth).
Once the airbrushed dried, I applied confectioner glaze to it all. I colored piping gel dark crimson and dripped it as needed (to make it more realistic)
Parts:
1 superior vena cava,
1 inferior vena cava,
1 aortic valve ,
1 pulmonary valve,
2 pulmonary veins

Have fun!