I tried looking at diagrams of how the filtration works but I’m hitting a roadblock in my head I won’t let it go.
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As blood passes through your kidney, some of the water and other small molecules flow into the kidney (more importantly, out of the blood vessel) through small holes barely big enough to fit those molecules. Blood cells & like are too big to go through those holes. That water+molecules that make it to the kidney are the urine.
Your body’s method of cleaning blood is kinda like cleaning out a refrigerator. It’s best to pull everything out of it and then put back the stuff it wants to keep.
The blood flowing through the kidney passes through the [glomerulus](https://en.wikipedia.org/wiki/Glomerulus_(kidney)) which has holes in it that are big enough to let everything smaller than a blood cell pass through. There are a lot of important stuff that got filtered out at this point. So as this filtrate passes through the tubules it absorbs the things it wants to keep back into the blood in the proportions it wants it. For instance you may have recently ate a banana and now have more potassium floating around in your blood than is healthy, but it’s also bad for you to get rid of all of it. So just a portion gets shipped off to the bladder.
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