A lot of our neurons are with us for our whole life. Early neuron degeneration is what causes Alzheimer's, Parkinsons, and similar disorders.

Not all neurons last a lifetime, and there are kinds that die off and are replaced, but a good chunk of them aren't meant to replicate anymore and so won't be freed of microplastics by bloodletting, and would cause serious problems if microplastics harm their normal processes.