Your turn
Each lab is one step from the video. Change things and see what happens.
Each lab isolates one part of the story. The models are simplified to show the idea; they are not medical tools.
Tap the buttons and slide the sliders.
Tap each stone. Most are the size of a grain of sand or a pea. Golf-ball stones are rare.
Stones range from sand-grain size to pea size; very rarely one reaches golf-ball size.
Tap a stone. Which one is as small as a grain of sand?
Press the pump. Each press is about one minute of work: half a cup of blood cleaned.
Kidneys filter about half a cup of blood a minute (≈150 quarts a day), yet only 1–2 quarts a day leave as urine.
Press the pump and watch the pee drip into the bladder.
Add minerals or add water. When does the water get too full, so crystals appear?
Crystals appear once the concentration passes saturation. Dilution brings it back below the line.
Add lots of bits. Now add water. What happens to the crystals?
A simplified model of a saturated solution.
Switch the risk factors on and off. Which one pushes the meter furthest?
Each factor either adds stone-forming minerals or takes away water. Low fluid intake matters most.
The sizes of the effects here are a teaching model, not real measurements.
Drag a small stone down the tube, then try a big one. If it gets stuck, use the shock waves.
Small stones usually pass; a large one can lodge in the ureter. Shock wave lithotripsy breaks it into passable pieces.
Drag the stone down the tube. A big stone gets stuck! Doctors can break it into little bits.
Severe pain, fever or blood in urine means see a doctor right away. This isn't medical advice.
Drink water, add a salty or a calcium snack. Watch the urine colour and the crystals.
Fluid dilutes urine; sodium raises urinary calcium; dietary calcium binds oxalate in the gut.
Tap the water glass. Does the pee get paler?
A simplified model. It shows how the body works; it isn't medical advice.
A kidney stone is a tiny hard stone that grows inside the body. Most are as small as a grain of sand. Some are as big as a pea.
You have two kidneys. They look like beans, and each one is about as big as your fist. They clean your blood all day. The mess they take out, and extra water, becomes pee.
Pee is water with tiny bits mixed in, like sugar in a drink. If you put in too much sugar, the extra sits at the bottom as little crystals. Pee can do that too!
The crystals stick together, very slowly, and grow into a stone. Not drinking enough water is one of the most common reasons. Hot days and very salty food can help stones grow too.
Plenty of water keeps the bits mixed in, so crystals are much less likely to form. When your pee is pale, like lemonade, you are drinking enough.
Kidney stones form when your urine carries more dissolved minerals than its water can hold. The main stone-forming minerals are calcium, oxalate and phosphorus. They are normally in urine and harmless at low levels.2 When there are too many of these particles and too little liquid, they start to stick together as crystals, and over months or years the crystals can grow into a stone.9
So anything that means less water or more minerals in urine raises the risk. Not drinking enough is one of the most common. Others are sweating a lot, very salty food, lots of animal protein, a family history and some medical conditions.1710 About 1 in 10 people will have a kidney stone at some point in their life.8
This page explains how the body works; it isn't medical advice.
A kidney stone is a hard, pebble-like piece of material that forms in one or both kidneys. Most are yellow or brown, and they can be smooth or jagged. They can be as small as a grain of sand or as large as a pea, and very rarely as big as a golf ball.1 Doctors call a kidney stone a renal calculus or nephrolith.1
To see where they come from, start with the kidneys themselves. You have two bean-shaped kidneys, each about the size of a fist, just below your ribs on either side of your spine. Together they filter about half a cup of blood every minute. They pull out waste, extra water and extra minerals, and that mix becomes urine.5 Each day your kidneys filter about 150 quarts of blood, but only 1 to 2 quarts end up as urine. Urine then flows down two thin muscular tubes called ureters into the bladder.514 Keeping the right balance of water and minerals is part of your body's homeostasis.
Think of sweet tea. Stir a spoonful of sugar into a glass of water and it disappears: it has dissolved. Keep adding sugar and at some point the water can't hold any more. Chemists call that point saturated. Any extra sugar stays on the bottom as crystals. If a liquid ends up holding even more than it normally can, it's supersaturated, and the extra is ready to come out as a solid.12
Urine works the same way. It's water with minerals, acids and salts dissolved in it.9 When it carries more calcium and oxalate than its water can keep dissolved, the extra comes out as tiny crystals. The crystals stick together and build up layer on layer. That's a kidney stone.910 It's the same idea as other crystals in nature, like the ice crystals in snowflakes or the carbon crystals in diamonds, only much smaller and messier.
Your body also has helpers that slow this down. Citrate, found in citrus drinks like lemonade and orange juice, can stop crystals from turning into stones.4
| Type | How common | What's behind it |
|---|---|---|
| Calcium oxalate | The most common type of all | Calcium joins with oxalate in urine; the tendency can run in families17 |
| Calcium phosphate | Common, less so than calcium oxalate | High calcium and phosphate in urine1 |
| Uric acid | Common | Urine that is too acidic; lots of meat, fish and shellfish can raise uric acid1 |
| Struvite | Less common | Can form after a urinary tract infection and grow quickly1 |
| Cystine | Rare | An inherited condition called cystinuria1 |
Men get kidney stones more often than women. In the US, about 11 percent of men will have a stone at least once in their life, compared with about 6 percent of women.1 Stones are most common in men in their 30s and 40s.9 The causes are the same as for everyone: too little water, too much salt or animal protein, heat, family history and some medical conditions. Scientists are still working out exactly why men are affected more often.13
Women get kidney stones from the same causes as men. The difference is how often. Stones used to be seen as mainly a men's problem, but the gap has narrowed: US surveys show the rate in men has stayed about the same, while the rate in women has risen (from about 6.5 to 9.4 percent in one national survey series).13 So yes, women can and do get kidney stones.
A stone can sit in the kidney without causing any trouble. Problems start when it moves into the ureter. A small stone may pass all the way through with little or no pain. A bigger one can get stuck, block the flow of urine and cause severe pain or bleeding.1 Around 80 percent of stones pass on their own, and a stone smaller than 4 millimetres may pass within one to two weeks.9
When a stone is too big, doctors have ways to help. One is shock wave lithotripsy, which sends shock waves from outside the body to break the stone into small pieces that can pass. Other methods use a thin scope to reach and remove the stone.4
Severe pain, a fever, or blood in urine means see a doctor right away.12 Treated in time, kidney stones rarely cause lasting damage.1
People who have had a stone can get a plan from their doctor based on the type of stone they had.4
Stones form when urine carries more stone-forming minerals, such as calcium, oxalate and phosphorus, than its water can keep dissolved. Not drinking enough liquid is one of the most common reasons: less water makes urine more concentrated, so minerals cluster into crystals.
Most are calcium stones, and calcium oxalate stones are the most common kind. Other types are uric acid stones, struvite stones (which can form after a urinary tract infection) and rare cystine stones, which come from an inherited condition.
No single food causes a stone on its own. Diets high in salt (sodium) and animal protein raise the risk, and for people who have had calcium oxalate stones, very high-oxalate foods such as spinach, rhubarb, nuts and wheat bran may matter. Too much added sugar can also play a part.
Drink enough liquid, mainly water, so your urine stays pale yellow or almost clear; drink more in hot weather or when you sweat a lot. Eating less salt and getting normal amounts of calcium from food also help. People who have had a stone should ask a doctor about a plan for their stone type.
In the US about 11 percent of men and 6 percent of women get a kidney stone at some point, and stones are most common in men in their 30s and 40s. The gap has been narrowing, because stones have become more common in women in recent decades.
They can run in families. A family history raises your risk, usually because many small gene differences combine with things like diet and how much you drink. A few rare conditions, such as cystinuria, are passed down directly.
Many small stones cause no symptoms. When a stone moves or blocks urine, it can cause sharp pain in the back, side, lower belly or groin, which may come in waves, plus blood in urine, nausea, or cloudy urine. Severe pain, fever or blood in urine means see a doctor right away.