There are no "millions" of 5mm plastic pieces in your bloodstream. That's about a rice grain. If there would be even a single one between 5mm and 1mm it would cause an almost immediate obstruction.
What part of "smaller than" was difficult to parse?
Microplastics can be defined as < 5mm (e.g. EPA uses that definition), doesn't mean the larger ones are in the bloodstream, or even less so, the brain.
But such "sizable" ones in the environment can and are be broken down, digested, shed smaller micro- and nanoplastics by the loads, and so on.
There are no "millions" of 5mm plastic pieces in your bloodstream. That's about a rice grain. If there would be even a single one between 5mm and 1mm it would cause an almost immediate obstruction.