The key process in the development of kidney stones is supersaturation.
- The urine carries salts, including calcium oxalate, uric acid, cystine, or xanthine.
- These salts can become extremely concentrated if there is not enough urine, or if unusually high amounts of crystal-forming salts are present.
- When salt concentration levels reach the point at which they no longer dissolve, these salts form crystals.
Different factors may be involved in either reducing urine amount, or increasing the levels of the salts.
Deficiencies in Protective Factors. Normally, urine contains substances that may protect against stone formation, including:
- Allow salt in the urine to be at higher-than-normal concentrations without forming crystals
- Prevent crystal formation
- Coat the crystals and prevent them from sticking to the surface of kidney tubes
Not having enough of these protective substances can cause stones.
Changes in the Acidity of the Urine. Changes in the acid balance of the urine can affect stone formation.
- Uric acid and cystine stones mainly form in acidic urine.
- Calcium phosphate and struvite stones increase in alkaline urine.
Causes of Calcium Stones
Review Date: 06/08/2010
Reviewed By: Reviewed by: Harvey Simon, MD, Editor-in-Chief, Associate Professor of Medicine, Harvard Medical School; Physician, Massachusetts General Hospital. Also reviewed by David Zieve, MD, MHA, Medical Director, A.D.A.M., Inc.