system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How do the kidneys know the difference between toxic metals and essential metals when filtering the blood and excreting the toxic ones into urine?
gpt
They don't know anything.  The kidneys are filters.  Every substance has different density, shapes and other properties, all of which can be used to filter things out or put them back in.  It's like sifting flour.  Anything too big or the wrong shape can't fit and thus gets sifted out.  


And sometimes they put things back in, such as minerls like iron which are need in small amounts but harmful in less amounts.  Those get put back in in the right amounts.  It's all biological eqivilants of sifters, sorting machines, and measuring cups, only on a much smaller scale.