After some trial and error I think I have cracked how to cleanly separate the two pieces of my current composition so that I can render out the blood cells moving while also putting the background on a separate layer so I can give off the illusion the cells are passing behind something not there.
|First test purely using a basic "use background" shader, didn't go so well.|
|Tweaking the render settings so that it would be visible and traced but not produce shadows.|
|A combined image that includes the picture above and a shot of the tunnel.|
Seeing how the tunnel turned out I do think I need ot tweak the lighting a little to add a hint of colour and vibrance.