Nephrons
The cortex and the medulla contain tiny filters called nephrons. These are the functional parts that clean up the blood, and there are about a million of them in each kidney. The nephrons clean by removing everything from the blood except blood cells and proteins, and then put back the useful substances.
The boy kid who needed the clean-up was the nephew (nephron) of the nurse who treated him.
This is how the nephron is positioned in the kidney, with its top part in the cortex and the lower parts in the medulla. There are usually about a million nephrons in each kidney.
It’s like cleaning out the fridge
When nephrons clean up the blood, it’s like cleaning a fridge – everything (all the substances apart from blood cells and proteins) has to come out of the fridge, so the inside of the fridge can be cleaned. Then the substances that are still in date and useful are put back into the fridge.
