A kidney stone is a solid piece of material that forms in the kidney out of crystals in the urine. The crystals normally remain tiny enough to travel through the urinary tract and out of the body in the urine without being noticed. This is because urine contains chemicals that prevent or inhibit the crystals from forming.
However, these inhibitors do not seem to work for everyone. In some people, the crystals stick together and continue to build up to form kidney stones.
A kidney stone may stay in the kidney or break loose and travel down the urinary tract. However, a large kidney stone may get stuck in a ureter, the bladder, or the urethra -- which would block the flow of urine and cause great pain.
For unknown reasons, the number of people in the United States with kidney stones has been increasing over the past 30 years, from 3.8 percent in the late 1970s to 5.2 percent in the late 1980s and early 1990s. Scientists believe that the increase may be related to diet and lifestyle.
Caucasians are more prone to developing kidney stones than African Americans, and the condition affects more men than women. The prevalence of kidney stones rises dramatically as men enter their 40s and will continue to rise into their 70s. For women, the prevalence of kidney stones will peak in their 50s. Once a person gets one kidney stone, other kidney stones are likely to develop.
What Do Kidney Stones Look Like?
Kidney stones may be as small as a grain of sand or as large as a golf ball. They may be smooth or jagged, and they are usually yellow or brown.
Kidney stones may contain various combinations of chemicals. The four most common types of kidney stones contain:
- Calcium
- Struvite
- Uric acid
- Cystine.
The most common type of stone contains calcium in combination with either oxalate or phosphate. These calcium stones are called calcium oxalate stones or calcium phosphate kidney stones.
(Click Types of Kidney Stones for more information.)
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