Very creative, yet morbid, little game. I initially thought the goal was to block the red blood cells, only to realize that, ironically, that's the opposite of what you want to do, as it slows down the collection of plaque.
A full version could track throughput of red blood cells, so that while getting bigger makes it easier, it's not the end goal itself. Then reduce the number of red blood cells and increase their speed, so the challenge is stopping enough of them to create a blockage. Different levels could have different speeds, different existing build ups of plaque, and branching or connecting paths. Just some ideas that come to mind.
Very neat little game, overall.