Kidney Stones

  • Introduction

    Kidney stones are hard, solid particles that form in the urinary tract. In many cases, the stones are very small and can pass out of the body without any problems. However, if a stone (even a small one) blocks the flow of urine, excruciating pain may result, and prompt medical treatment may be needed.

    Urine is formed in the kidneys. The kidneys filter out fluids and waste from the body, producing urine. As the urine passes through the kidneys, it becomes more concentrated. From the kidneys, urine flows through thin tubes called ureters into the bladder. The bladder's stretchy walls expand to store the incoming urine until it leaves the body through a tube called the urethra.

    The two kidneys are located deep behind the abdominal organs, below the ribs and toward the middle of the back.

    The kidneys are responsible for removing wastes from the body, regulating electrolyte balance and blood pressure, and stimulating red blood cell production.
    Click the icon to see an image of the urinary tract.

    Types of Kidney Stones

    Occasionally, high levels of chemicals in the urine form into crystals. Eventually these crystals become large enough to form stones in the kidney, a condition called nephrolithiasis. Stones (calculi) may also form in the ureter or the bladder. Combinations of minerals and other chemicals, some derived from a person's diet, make up the salts in these stones.

    Click the icon to see an image of the kidney stones.

    Calcium Stones. About 70 - 90% of all kidney stones are made of calcium, usually combined with oxalate, or oxalic acid. A number of common vegetables, fruits, and grains contain oxalate.

    About 6% of calcium stones are made of calcium phosphate (called brushite).

    Uric Acid Stones. Uric acid is responsible for close to 10% of kidney stones. It is the breakdown product of purines, nitrogen compounds found in the body and in certain foods. Uric acid enters the bloodstream, and then passes primarily into the kidneys. From the kidneys, uric acid leaves the body in the urine. Often, uric acid stones occur with calcium stones.

    Struvite Stones. Struvite stones are made of magnesium ammonium phosphate. They are almost always associated with certain urinary tract infections. Worldwide, they account for up to 30% of all kidney stones. In the United States, however, fewer than 15% of all stones are struvite. Most struvite stones occur in women. The rate of these stones may be declining in America, perhaps because of better control of urinary tract infections.

    Cystine Stones. A build-up of the amino acid cystine, a building block of protein, causes 1% of kidney stones in adults and up to 8% of stones in children. The tendency to form these stones is inherited. Cystine stones grow rapidly and tend to recur. If not treated promptly, they can eventually lead to kidney failure.

    Xanthine Stones. Other kidney stones are composed of xanthine, a nitrogen compound. These stones are extremely uncommon and usually occur as a result of a rare genetic disorder.

    Click the icon to see an animation about kidney stones.