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. Glomerular filtration and tubular transport perform distinct roles in a nephron. A selective filtration barrier retains blood cells and most large proteins, while water and many small solutes enter the tubule. Reabsorption and secretion then modify the filtrate. The routes separate filtration from later tubular transport, which together determine what remains in the fluid.
The human body · Discover & try
What stays in blood, and what becomes urine?
Reading level 5 of 5 · AdvancedChange
Word help
Nephron
A kidney unit containing a filter and tubule.
Reabsorption
Returning a filtered substance to blood.
Watch · Try · Explain
See how it works
The interactive activity is loading.
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.
Follow the sources
Source material checked 2026-10-29. Explanations and activities by Nepedia. Source links do not imply endorsement.
Keep following the idea
Next discovery →