Kidney stones

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Urine is a solution of various waste substances the body doesnâ€™t need. Normally, there is plenty of water to keep these substances dissolved. Sometimes these substances are present in such high concentration that crystallization occurs. This is more likely when the body is short of water (dehydration) and the urine becomes concentrated.

Once a small crystal has formed, it acts as a seed for further crystallization and, in this way, stones (calculi) can grow to a large size, sometimes completely filling the upper part of the urine collecting system of the kidney. Because of the shape of the collecting system, these stones are called â€˜staghornâ€™ calculi.

Kidney stones can move to lower down in the urinary system: to the ureters which carry urine down to the bladder, or in the bladder itself. Small stones (the size of a grain of sand) may be passed out through the tube from the bladder to the exterior (the urethra). Bigger stones may not be passed out, and can get stuck anywhere along the urinary system, depending on their size.