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50 free Kidneys trivia questions with answers — science & nature quiz, new questions added Aug 2026.
The kidneys are two bean-shaped organs about the size of a fist, and they do far more than make urine. This quiz covers the whole story: where they sit and how they are protected, the million nephrons packed into each one, the loop of Henle and why a camel's is longer than yours, the 180 litres of filtrate a day, and the hormones the kidneys make that control blood pressure, red blood cells and vitamin D. From there it moves into medicine and history: Willem Kolff building the first dialysis machine from sausage casings and juice cans in occupied Holland, the 1954 Herrick twin transplant that won Joseph Murray a Nobel Prize, Richard Bright's disease, kidney stones dated to 4,800 BC, the 2024 pig-kidney xenotransplants, and the one country where selling a kidney is legal. There is also room for the lighter side, from Roman urine taxes and steak and kidney pie to why ancient Egyptians left the kidneys inside their mummies. Easy questions suit anyone who remembers school biology; the expert tier is for nurses, medics and pub-quiz specialists. Every answer was checked against Wikipedia's articles on the kidney, nephron, dialysis, transplantation and related topics, and the supporting sentence is attached to each question.
30 of 50 questions with answers and explanations. Play the quiz
Q 01Roughly how long is an adult human kidney?
12 cm
About the length of a smartphone; a Danish study measured a median of 11.2 cm on the left and 10.9 cm on the right.
Q 02Which tube carries urine from each kidney to the bladder?
Ureter
The urethra is the exit tube from the bladder; the two ureters feed it from above.
Q 03What is the structural and functional unit of the kidney called?
Nephron
Each adult human kidney has around a million of them; a mouse kidney gets by with about 12,500.
Q 04Approximately how many nephrons does each adult human kidney contain?
1 million
By early chronic kidney disease that number is roughly halved, similar to the loss between young adulthood and one's seventies.
Q 05What share of the heart's output flows through the kidneys in an adult?
About 20-25%
That is a huge share for two organs weighing a few hundred grams between them, which is why blood-pressure problems hit the kidneys hard.
Q 06About how much filtrate do an adult's kidneys produce each day, most of it reabsorbed?
180 litres
Only around one to two litres leaves the body as urine; the rest is reclaimed along the tubules.
Q 07The kidneys convert a precursor of which vitamin into its active form, calcitriol?
D
Calcitriol boosts calcium absorption in the gut, which is one reason kidney failure leads to bone disease.
Q 08Which hormone, secreted mainly by the kidneys, stimulates red blood cell production in the bone marrow?
Erythropoietin
Its synthetic form became notorious as a doping product in cycling; the Festina affair exposed it before a test even existed.
Q 09The prefix nephro- comes from the word for kidney in which language?
Greek
The adjective renal comes from the Latin renes instead, which is why kidney doctors are nephrologists who treat renal disease.
Q 10The kidneys sit in which space of the body?
Retroperitoneal
They lie behind the peritoneal lining of the abdomen, high up on either side of the spine at a slight angle.
Q 11Which pair of ribs partially protects the upper parts of the kidneys?
11th and 12th
The right kidney sits slightly lower than the left because the liver takes up room above it.
Q 12The loop of Henle is named after an anatomist of which nationality?
German
Friedrich Gustav Jakob Henle's loop builds the salty gradient in the kidney medulla that lets urine be concentrated.
Q 13Which animal's loop of Henle, twice a human's length, concentrates urine to about 2,800 mOsm?
Camel
An Australian hopping mouse goes further still, with a 5.2 mm loop that makes its medulla about 9,000 mOsm.
Q 21Which is the most common type of kidney stone?
Calcium oxalate
Oxalate is about 15 times stronger than calcium at promoting these crystals, which is why spinach and rhubarb get a bad name.
Q 22A kidney stone found in an ancient Egyptian mummy of a teenage boy was dated to roughly when?
4,800 BC
Surgery to remove stones was described by Sushruta in India around 600 BC, and the Hippocratic Oath tells physicians to leave cutting for stones to specialists.
Q 23Large branching kidney stones that fill the renal calyces are named after which animal's antlers?
Stag
These staghorn stones are nearly always struvite and only form in the presence of urease-producing bacteria.
Q 14The cup-shaped structure that surrounds the glomerulus is named after which anatomist?
Bowman
Bowman's space collects the filtrate; normally only blood proteins, blood cells and platelets are held back.
Q 15The specialised cells whose foot processes form part of the glomerular filter are called what?
Podocytes
The name comes from the Greek for foot; the interlocking foot processes leave slits just narrow enough to keep proteins in the blood.
Q 16Which cells in the kidney release the enzyme renin when blood flow to the organ drops?
Juxtaglomerular
Renin kicks off the renin-angiotensin system, the blood-pressure cascade targeted by ACE inhibitors.
Q 17ACE, which converts angiotensin I to II, is found predominantly on blood-vessel cells of which organ?
Lungs
The kidney starts the cascade with renin, the liver supplies angiotensinogen, and the lungs finish the job.
Q 18The glands atop the kidneys that produce adrenaline and cortisol are also called what?
Suprarenal
The name, coined by Jean Riolan the Younger in 1629, is Latin for above the kidney.
Q 19The pigment that gives urine its usual pale yellow colour is called what?
Urobilin
It is a final waste product of breaking down the heme in old red blood cells.
Q 20Which Roman emperor is credited with the saying 'money does not smell' after taxing the urine trade?
Vespasian
His name still survives in the words for public urinals in France, Italy and Romania.
Q 24ESWL, a non-invasive treatment that breaks up kidney stones from outside the body, uses what?
Shock waves
The full name is extracorporeal shock wave lithotripsy; lithotripsy literally means stone crushing.
Q 25In the developed world, roughly what is the lifetime risk of getting a kidney stone?
10-15%
In the Middle East it can reach 20-25%, and about half of sufferers get another one within ten years.
Q 26Who built the first working dialysis machine, in the occupied Netherlands in 1943?
Willem Kolff
He later moved to Utah, where his lab produced the Jarvik artificial heart implanted in Barney Clark in 1982.
Q 27Kolff's first prototype dialyzer was built from orange juice cans, auto parts and what else?
Sausage casings
The casings served as the semi-permeable membrane; his first patient to be saved was a 67-year-old woman in 1945.
Q 28The world's first outpatient dialysis facility, opened by Belding Scribner in 1962, was in which US city?
Seattle
Scribner's Teflon shunt made repeated dialysis possible; the centre was later renamed the Northwest Kidney Centers.
Q 29Dialysis is usually started once roughly how much kidney function has been lost?
85-90%
That corresponds to a glomerular filtration rate below 15; life expectancy on dialysis is typically 5-10 years, though some live 30.
Q 30The first successful kidney transplant, in Boston in 1954, was performed between what kind of pair?
Identical twins
Ronald Herrick gave a kidney to his brother Richard; sharing the same genes sidestepped rejection years before immunosuppressants existed.