Kidneys start by filtering small substances from blood into a tiny tube. Useful substances can then return to the blood. A blood cell normally stays in the blood. Filtered fluid is not all waste: only what remains after these exchanges contributes to urine. Blood cells and most large proteins normally stay in the blood.
The human body · Discover & try
What stays in blood, and what becomes urine?
Reading level 1 of 5 · EarlyChange
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Nephron
A kidney unit containing a filter and tubule.
Reabsorption
Returning a filtered substance to blood.
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See how it works
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A simplified nephron route map. Particle counts do not represent real filtration rates, concentrations or individual kidney function.
What to look for
- It can enter filtered fluid, then return to blood in this normal route.
- The cell stays in blood and does not pass into the tubule.
- This marker stays with fluid that reaches urine; most filtered water actually returns to blood.
Explain what you noticed
A large protein approaches the normal filter in this model. Which route is most appropriate?
Keep this idea: Kidneys filter fluid, then selectively return and remove substances before urine leaves.
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Source material checked 2026-10-29. Explanations and activities by Nepedia. Source links do not imply endorsement.
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