It is not unusual for tiny crystals to form in the urine of healthy people, as many of the substances eliminated by the kidneys are only just soluble in water. A change in the acidity of urine, for example, can change the chemical environment of the urine enough to cause crystals to form. Provided these crystals remain small enough, they are flushed out without causing any symptoms or trouble. Some people do however have an increased tendency to form urine stones, either starting within the kidney and ending up in the bladder, or forming within the bladder itself. A structural abnormality of the bladder such as blind pouch might encourage this by producing a 'stagnant' area that does not drain properly when passing urine. Another complicating factor is infection of the urine, which can be due to and encourage the further growth of bladder stones. In older men an enlarged prostate can encourage the growth of a bladder stone by restricting the proper and full drainage of the bladder.

Whereas a stone of only a few millimetres in size can easily cause great pain if it gets held up during its travel through the narrow ureter - the tube linking the kidney and bladder (renal colic), a bladder stone can be several centimetres in size and not necessarily give symptoms, providing it doesn't block the emptying of the bladder itself, which is of course much more roomy.

Many people walk around with a bladder stone for years without noticing. But if the bladder stone irritates the mucosa (soft lining) of the bladder, blood may appear in the urine. Typically the blood will appear towards the end of urination.

A bladder stone may also cause pain when urinating. If the stone gets stuck at the entrance to the urethra, the jet of urine will suddenly stop in the middle of urinating.

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