Here is the background that i created:


I tried to create some details on the blood vessel, (for example, the concavo-convex surfaces,) but later on i found it's really complex to create neither in Flash or in Illustrator. That's why i give it up halfway and decide to create such a scene - which is simple but really show an idea of this bloody world.
And here is my final scene:
These blood vessels are keep moving so it looks like the monster is breathing.
I may add more details later to perfect it.