Your kidneys are two bean-shaped organs, each about the size of your fist. They're located in back of your abdomen on each side of your spine, and their main function is to remove excess fluid, unneeded electrolytes and wastes from your blood in the form of urine. The ureters carry urine from your kidneys to your bladder, where it's stored until you eliminate it from your body.
Kidney stones form when the components of urine — fluid and various minerals and acids — are out of balance. When this happens, your urine contains more crystal-forming substances, such as calcium and uric acid, than the available fluid can dilute. At the same time, your urine may be short of substances that keep crystals from sticking together and becoming stones. Kidney stones are also prone to develop in highly acidic or highly alkaline urine.
Problems in the way your system absorbs and eliminates calcium and other substances create the conditions for kidney stones to form. Sometimes, the underlying cause is an inherited metabolic disorder or kidney disease. Gout promotes specific types of kidney stones, as does inflammatory bowel disease. So do some drugs, including furosemide (Lasix), used in treating heart failure and high blood pressure; topiramate (Topamax), an anti-seizure drug; and indinavir (Crixivan), which is used to treat human immunodeficiency virus, the cause of AIDS.
It's common, however, for kidney stones to have no definite, single cause. A number of factors, often in combination, create the conditions in which susceptible people develop kidney stones.
Most kidney stones contain crystals of more than one type. Determining the type that makes up the bulk of the stone — usually a combination of calcium compounds — helps identify the underlying cause.
The best preventive approach after your first kidney stone also depends partly on the stone's composition.
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Calcium stones. Roughly four out of five kidney stones are calcium stones, usually in the form of calcium oxalate. Oxalate is found in some fruits and vegetables, but the liver produces most of the body's oxalate supply. Dietary factors, high doses of vitamin D, intestinal bypass surgery and several different metabolic disorders can increase the concentration of calcium or oxalate in urine.
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Struvite stones. Found more often in women, struvite stones are almost always the result of urinary tract infections. Struvite stones may be large enough to fill most of a kidney's urine-collecting space, forming a characteristic stag's-horn shape.
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Uric acid stones. These stones are formed of uric acid, a byproduct of protein metabolism. You're more likely to develop uric acid stones if you eat a high-protein diet. Gout also leads to uric acid stones. Certain genetic factors and disorders of the blood-producing tissues also may predispose you to the condition.
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Cystine stones. These stones represent only a small percentage of kidney stones. They form in people with a hereditary disorder that causes the kidneys to excrete excessive amounts of certain amino acids (cystinuria).
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Kidney stones (renal lithiasis) : Overview (PDF/PPT Collection) 
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