A kidney stone (nephrolithiasis) is a solid piece of material made from chemicals that form in the kidney. It's made from substances that normally dissolve in the urine. Each year, more than half a million people go to the emergency room with kidney stone problems.

Kidney stones often occur between age 20 and 70 and are more common in males than females. They are also more common in non-Hispanic white people than in people of other ethnic groups. If you get kidney stones, you may be more likely to get them in the future.

Your urinary tract is made up of several parts:

• Kidneys. These are two bean-shaped organs about the size of your fist.
• Ureters. These are two narrow muscular tubes that drain urine from the kidneys into the bladder.
• Bladder. This organ temporarily stores urine.
• Urethra. This is the tube that empties urine from the bladder.

The kidneys make urine by removing extra water and waste from your blood. They also help your body balance certain chemicals in your blood.

Kidney stones are made up of chemicals in your urine. These include:

• Calcium
• Oxalate
• Phosphate
• Uric acid (urate)
• Cystine
• Struvite

Stones begin as crystals that build up inside the kidney. Small crystals can pass from your body through the urine without you ever knowing about them. When the crystals grow larger and become stones, they can get stuck in the ureter and block the flow of urine. This causes infection and pain.

About 3 in 4 kidney stones contain calcium plus either oxalate or phosphate. A less common type of stone is composed of struvite. It's caused by urinary tract infections. It's made of magnesium ammonium phosphate. Struvite stones make up 1 to 1.5 in 10 kidney stones. About 1 in 10 of the remaining stones are made of uric acid. Uric acid is made from purine. This is a nitrogen compound found in protein and made in your liver. Fewer than 3 in 100 stones are made of the amino acid cystine. This type is rare and tends to run in families.

Stones can be as small as a grain of sand or as large as a golf ball.

It's not always completely clear what causes kidney stones to form. Some people are more likely to get them because of their family history of stones. In many cases, they are caused by having too many salts in your urine. These salts are calcium oxalate, uric acid, and cystine. Salts can build up in the kidney when you don't have enough urine, or you have higher levels of the salts than normal. When the salts build up to a certain point, they no longer dissolve. They form crystals instead. Normally, the urine contains chemicals and enzymes that keep crystals from forming, or from sticking to the inner surface of the kidney.

In other words, changes in the way your body uses food and beverages may increase your chances of growing crystals and getting kidney stones. Some types of kidney stones run in families (hereditary).

There are no true "warning signs" for kidney stones. Small stones are usually passed unnoticed. Large stones often remain undetected until they become lodged in the ureter and you have severe and sudden pain in the back or lower abdomen. The pain is usually described as a sharp and cramping pain in the back, the side, or the lower belly (abdomen). The pain may spread to the groin. There may also be blood in the urine, nausea, and vomiting. If the stone is too large to pass, the muscles of the ureter tighten trying to squeeze the stone into the bladder. Fever or chills usually mean there is an infection. Sometimes stones stay in the kidneys and grow larger.

Most kidney stones can be diagnosed with an X-ray or ultrasound. These images tell your doctor the stone's size and location. Blood and urine tests help find any abnormal substance that might help stones form. Some imaging tests can see if fluid is backing up in the kidneys that could cause damage.

Your doctor may decide to scan the urinary system using a CT scan or a special X-ray test called an intravenous pyelogram. The results of these tests and the others above help your doctor figure out the diagnosis and the right treatment for you.

In most cases, treatment means letting the stone pass naturally. Your doctor may give you medicine for pain relief. Then they will add fluids until the ureter builds up enough pressure to push the stone out. You will usually be asked to catch the stone in a strainer so it can be looked at in the lab. If the stone hasn't passed in 8 to 12 hours, you may need to see a urologist for more treatments.

For stones that don't pass on their own, other treatments are used:

• Extracorporeal shock wave lithotripsy (ESWL). This treatment uses an ultrasound machine to send shock waves through the skin and body tissue toward the stone. The shock waves break the stone into small pieces that can pass from your body in the urine.
• Percutaneous nephrolithotomy. This is surgery to remove stones that are too large or that can't be reached with ESWL. A surgeon makes a small incision in your back and makes a tunnel to the kidney. The stone is then taken out through a nephroscope. If the stone is very large, the surgeon can put an energy probe in to break the stone into smaller pieces and then remove the pieces. If you have this surgery, you will need to stay in the hospital for 2 to 3 days.
• Ureteroscopic stone removal. The surgeon puts a fiber optic tool into the urethra and the bladder. They then move it into the ureter. After the stone is found, the surgeon removes it with a cage-like basket device or shatters it with laser beams or shock waves. Often, the surgeon puts a stent in the ureter to keep the tube widened and ease passing of the stone pieces.

Preventing kidney stones is important, especially if you've had a kidney stone in the past. This is because you may be at a higher risk to form another. The National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK) says that the preventive treatment your doctor gives you will depend on the type of stone you have. For example, a medicine that helps prevent calcium stones will not work if you have a struvite stone. The diet changes that help prevent uric acid stones may have no effect on calcium stones. So careful study of the stone will help guide your treatment.

If you have obesity, losing weight may help.

The most important part of prevention is getting plenty of fluids. This dilutes your urine and increases urination. More trips to the bathroom help remove extra chemicals from the urine and kidneys. This lowers the chances of stone formation.

The NIDDK recommends that you drink at least 6 to 8 8-ounce glasses of fluid every day, unless your doctor tells you otherwise. Their drink of choice? Water. Water is available to everyone and is inexpensive.

People who form kidney stones were once told to stay away from dairy products and other foods that contained a lot of calcium. Recent studies have shown that foods high in calcium actually help prevent stones, according to the NIDDK. But calcium supplements may increase the risk of developing stones. It's important to talk with your doctor before taking calcium supplements.

For some people, eating less protein, especially protein from meat, may help prevent certain stones. Protein can increase uric acid, calcium, and oxalates in the urine. Protein also reduces citrate.

If you are at risk for uric acid stones, you may be helped by eating fewer foods that contain purines. These foods include sardines, yeast, and organ meats.

If you are a person who absorbs a lot of oxalate, you should not drink large amounts of tea, because it has a lot of oxalate. You should also drink less or no alcohol. Other foods to pass up include chocolate, beets, coffee, cola, nuts, rhubarb, spinach, strawberries, and wheat bran.

Your doctor may prescribe medicine to help prevent kidney stones. The medicines control the amount of substances in the urine that form crystals or prevent the infections that can lead to stone growth.

You should also limit how much salt (sodium) you get each day. Salt is another chemical that must leave the body through the urine. The more chemicals that are in the urine, the more likely you are to form a stone.

Things that may increase your chance of developing kidney stones are:

• A family history of kidney stone formation
• Not drinking enough water
• A history of previous kidney stones
• Eating too much protein, sodium (salt), or sugar
• Having extra weight or obesity
• Intestinal surgery
• An inherited condition that causes the body to absorb too much calcium
• A low level of citrate in the urine. This may help calcium stones form.
• Overactive parathyroid glands
• Urinary tract infections
• Gout
• Bowel disease
• High blood pressure