Here's the heart of my dragon, along with its lungs and the air sac that sits behind the trachea (there will be seven air sacs, as in a bird). Next time, I'll explain about the air sacs and how I get around the problem of simulating membranes with paper mache.
Usually I use a lot more Payne's grey for corrosion cast blood vessels, but in this case I've used mainly cadmium red so they'll stand out in the dark chest cavity. I have cheated a bit with the blood vessels; as you can see they're modelled off a human circulatory system, not an avian one. I didn't have a good anatomical diagram showing how a bird's heart and lungs are connected, so I figured that since birds have a four-chambered heart like humans the plumbing system couldn't be all that different.