Let's say you keep dripping slightly muddy water into a bucket. Over time, the mud will settle and accumulate in the bucket, while the clearer water will overflow. Now suppose you cut a slit at the base of the bucket. Now the mud will flow out through it and the water in the bucket will become less muddy, even though new muddy water is still dripping in. Here the bucket is your bloodstream, the slightly muddy water is your food, and the mud is microplastic.