It enters the body physically, then spreads around the body physically, unaffected by chemical processes. The small pieces are small enough to enter the bloodstream, and from there the brain.
If that's true, then I'm even more confused regarding how we assimilate and absorb nutrients into the body. I thought these broken down nutrients were microscopic and selectively absorbed, which suggests that our bodies naturally contain all sorts of potential contaminates that are not chemically broken down like nutrients are.
I'm feeling a little skeptical.
The brain "uses" a big part of all the blood in the body. Over 10 percent from what I remember. Could also be that for some reason the plastic can only traverse the blood brain barrier in one direction and so it accumulates in the brain.