Filtering the blood
Blood carries wastes from every cell. Something must clean the blood.
- The kidney: a filter.
- Blood flows in.
- Tiny units called nephrons filter it.
- Most filtered liquid goes back to the blood.
- What remains leaves as urine.
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The rule
The kidneys clean the blood. Each kidney holds many filtering units called nephrons.
Capillaries are the thinnest blood tubes. In a nephron, a cluster of them filters the blood. A cup-shaped Bowman’s capsule collects the filtrate. That is the liquid that passes the filter. Blood cells are too big to be filtered. They stay in the blood.
The filtrate then flows along a coiled tube. Here the nephron takes things back.
A nephron filters blood. It takes back glucose, amino acids, salts and much of the water. What is left leaves as urine. Amino acids are the small parts that make up protein. The body needs these useful things, so the nephron takes them back.
Urea is a waste with nitrogen. The body makes urea when it breaks down protein. Uric acid is another waste with nitrogen. Both leave in urine. How much water is taken back depends on the body’s needs.