Kidney stones are hard objects, made up of millions of tiny crystals. Most kidney stones form on the interior surface of the kidney, where urine leaves the kidney tissue and enters  the urinary collecting system . Kidney stones can be small, like a tiny pebble or grain of sand, but often are much larger.

The job of the kidneys is to maintain the body's balance of water, minerals and salts. Urine is the product of this filtering process. Under certain conditions, substances normally dissolved in urine such as calcium, oxalate, and phosphate, become too concentrated and can separate out as crystals. A kidney stone develops when these crystals attach to one another, accumulating into a small mass, or stone.

Kidney stones come in a variety of mineral types.

  1. Calcium Stones: Most kidney stones are composed of calcium and oxalate. Many people who form calcium containing stones have too much calcium in their urine, a condition known as hypercalciuria. There are several reasons why hypercalciuria may occur. Some people absorb too much calcium from their intestines. Others absorb too much calcium from their bones. Still others have kidneys which do not correctly regulate the amount of calcium they release into the urine. There are some people who form calcium oxalate stones as a result of too much oxalate in the urine, a condition known as hyperoxaluria. In some cases, too much oxalate in the urine is a result of inflammatory bowel disease, such as Crohn's disease or ulcerative colitis, or other times it may be a consequence of prior intestinal surgery. Calcium phosphate stones, another kind of calcium stone, are much less common than calcium oxalate stones. For some people, calcium phosphate stones form as a result of a medical condition known as renal tubular acidosis. 

  2. Struvite Stones: Some patients form stones that are composed of a mixture of magnesium, ammonium, phosphate, and calcium carbonate, which is known as struvite. These stones form as a result of infection with certain types of bacteria that can produce ammonia. Ammonia acts to raise the pH of urine which makes it alkaline and promotes the formation of struvite. 

  3. Uric Acid Stones: Uric acid is produced when the body metabolizes protein. When the pH of urine drops below 5.5, urine becomes saturated with uric acid crystals, a condition known as hyperuricosuria. When there is too much uric acid in the urine, stones can form. Uric acid stones are more common in people who consume large amounts of protein, such as that found in red meat or poultry. People with gout can also form uric acid stones. 

  4. Cystine Stones: Cystine stones are rare, and they form only in persons with an inherited metabolic disorder that causes high levels of cystine in the urine, a condition known as cystinuria.

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