Which organ filters waste out of your blood?

Your kidneys process something like 180 litres of fluid a day, which is roughly forty times the amount of blood you actually have - the same blood goes round and round, being cleaned each time. Of those 180 litres, about 178 are reabsorbed. The rest leaves as urine.

The filtering is done by about a million tiny units per kidney called nephrons, each with a knot of capillaries that pushes water and small molecules out of the blood, followed by a long tubule that reclaims almost all of it - along with the glucose, salts and amino acids worth keeping. The design is wasteful-looking and enormously precise: it throws nearly everything out and then carefully takes back what it wants.

They do more than plumbing. Kidneys set your blood pressure, regulate the acidity of your blood, activate vitamin D, and produce the hormone that tells your bone marrow to make more red blood cells - which is why kidney failure causes anaemia, and why a synthetic version of that hormone became one of the best-known drugs in cycling.