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Kidney Stones

What Are Kidney Stones?

Last modified: 
16/04/2013 - 13:51

Contributing Author: Guy Slowik FRCS

Kidney stones are solid, stone-like lumps that form within the kidneys. They can be as small as a grain of sand or as large as the size of a golf ball (sometimes even larger). Most stones are less than a third of an inch in diameter.

The stones are actually accumulations of mineral salts and other substances. Similar stones also can form in most other portions of theurinary tract.

Many of these stones are small enough to pass out of the body during urination, but some larger stones may partially or fully block the normal flow of urine. Kidney stones can cause intense pain and other symptoms.

About 10 percent of people will experience a kidney stone at some time during life. Kidney stones are a common cause of blockage of the urinary tract in adults, accounting for 1 of every 1,000 hospitalizations.

What Are Kidney Stones Made Of?

Kidney stones consist of:

  • A center that consists of crystal-like substances
  • A surrounding region that is composed of layers

Kidney stones are composed of different chemical substances. Each of the major types is named for its main chemical ingredient. They are:

Calcium Stones

Calcium stones are the most common type of kidney stones, accounting for 75 to 85 percent of all stones.

  • Most of these stones consist of calcium oxalate, or a combination of calcium oxalate and phosphate.
  • Their formation is linked to high levels of urinary calcium, or a combination of calcium and uric acid, in the urine.

Uric Acid Stones

Uric acid stones are relatively uncommon, accounting for 5 to 8 percent of all stones.

  • A high level of uric acid in the blood and urine can cause this type of stone.
  • They can form if a person's urine becomes concentrated with certain dissolved substances that may make it more acidic.

Cystine Stones

Cystine stones are rare, accounting for less than one percent of all kidney stones.

  • These stones result from an inherited condition that causes an increase in the amount of cystine (an amino acid) in the urine.
  • Because cystine cannot dissolve easily in water, it cannot be reabsorbed from the urine into the blood. This increases the chances that a stone will form.

Struvite Stones

Struvite stones make up 10 to 15 percent of all kidney stones. Because these stones can be quite large, they can cause serious symptoms.

  • Struvite stones are formed because of infection by specific bacteria, which throws off the balance of acid in the urine.
  • These stones are mainly composed of ammonium and magnesium phosphate and resemble hard crystals.

Faces About Kidney Stones

  • Problems from kidney stones are fairly common, accounting for 1 of every 1,000 hospitalizations.
  • About 5 percent of all people have kidney stones.
  • There are four types of kidney stones: calcium stones, uric acid stones, cystine stones, and struvite stones.
  • Most of the time, there's no way to tell why the most common type of stones - calcium stones - have formed, although experts believe water intake and diet may play a role.
  • Kidney stones are most commonly found among people in the United States and in other technologically developed countries, while bladder stones are more common among people in developing countries.
  • Excruciating pain is the main symptom of a kidney stone. But not all kidney stones cause symptoms.
  • The presence of a urinary tract infection can increase the severity of stone disease and make treatment more difficult.
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Kidney Stones

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From Andrew Maynard - Chair of the University of Michigan Department of Environmental Health Sciences, with help from David Faulkner - 2013 Master of Public Health graduate.