Each kidney cleans the blood using microscopic filtering units called nephrons. About how many nephrons are there in a single kidney?
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Answer
About 1 million nephrons
Together the two kidneys filter around 180 litres of fluid every day, yet only a litre or two leaves the body as urine.
How to estimate it
A kidney weighs about 150 grams, so its volume is about 150 cubic centimetres. A nephron is mostly a thin tube, roughly 5 cm long and 0.05 mm wide. Its volume is 3.14 × 0.0025² × 5, about 0.0001 cubic centimetres. 150 / 0.0001 = 1.5 million, and since blood vessels also take space, call it 1 million. The textbook figure is about 1,000,000 nephrons.
Source: US National Institute of Diabetes and Digestive and Kidney Diseases
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