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ELI5 Yahaaa

Throw it all out, take back what you need

What Do Kidneys Actually Do?

Kidneys do not pick the rubbish out of your blood. They dump almost everything and then reclaim the good parts.

All your blood passesthrough, many times adayA million tiny filtersinsideAlmost everything smallis dumpedThen the useful partsare taken back
Filter first, sort afterwards.

  1. Step 1 of 5

    Picking out waste one piece at a time would be hopeless.

    Pick out the bad Hundreds of things torecogniseDump and reclaim Only a few things torecognise

    There are hundreds of different wastes and no single test for them. Recognising each one individually would need hundreds of different machines.

  2. Step 2 of 5

    So the filter only asks how big something is.

    One of a millionfilter units Small things through,big things stay

    Blood is pushed against a fine mesh. Blood cells and big proteins are too large to pass; water, salt, sugar and every small waste go straight through.

  3. Step 3 of 5

    That throws out far too much.

    Filtered each dayabout 180 litresActually passed outabout 1.5 litres

    About 180 litres a day cross the filters — some 60 times the total water in your body. Almost none of it can be allowed to leave.

  4. Step 4 of 5

    The long tube afterwards takes back what matters.

    1 Fluid enters along windingtube2 Water and saltare pulled back3 Returned to theblood

    Every filter drains into a winding tube that reclaims all the sugar, most of the salt and 99% of the water, pumping them back into the blood.

  5. Step 5 of 5

    How much you take back is how you stay balanced.

    Plenty of water Reclaim less — pale andplentifulShort of water Reclaim almost all —dark and scant

    Drink a lot and the tube reclaims less, so you pass pale water. Get hot and thirsty and it reclaims almost everything, so what leaves is dark and small.

    Your kidneys are about 0.5% of your body weight but take around 20% of the blood your heart pumps.

The short version

Kidneys clean blood by filtering nearly everything small out and then deliberately reclaiming the water, salt and sugar you need.

Try it yourself

Notice the colour after a hot afternoon and after a big drink. You are watching the reclaim setting change.

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