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. Filtration alone would lose many useful substances. Tubules therefore reabsorb water, nutrients and ions while also secreting selected substances from surrounding blood. Blood cells and most large proteins normally remain behind the glomerular barrier. The combination regulates the fluid eventually leaving as urine; urine is not simply blood poured through a single coarse sieve.
The human body · Discover & try
What stays in blood, and what becomes urine?
Reading level 4 of 5 · FluentChange
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Nephron
A kidney unit containing a filter and tubule.
Reabsorption
Returning a filtered substance to blood.
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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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