glomerulus
"The glomerulus is like a sieve which takes blood, filters it, and makes sure large molecules like proteins and cells don't get in to your urine. If there is a problem with the glomeruli, they get leaky, all sorts of stuff gets in to the urine that's not supposed to be there, and one of those things is a smaller protein called albumin."
The liquid goes through and the big stuff stays and gets moved back into the blood stream.
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