On occasion, calcium, magnesium or uric acid salts in urine may crystallize and precipitate in the renal pelvis, forming renal calculi (calculus=little stone), or kidney stones. Most calculi are under 5 mm in diameter and pass through the urinary tract without causing problems. However, larger calculi can obstruct an ureter and block urine drainage. Increasing pressure in the kidney causes excruciating pain, which radiates from the flank to the anterior abdominal wall on the same side. Pain also occurs when the contracting ureter wall closes in on the sharp calculi as they are being eased through an ureter by peristalsis.
Predisposing conditions are frequent bacterial infections of the urinary tract, urine retension, high blood levels of calcium and alkaline urine. Surgical removal of calculi has been almost entirely replaced by shock wave lithotripsy, a noninvasive procedure that uses ultrasonic shock waves to shatter the calculi.The pulverized, sandlike remnants of the calculi are then painlessly eliminated in the urine. People with a history of kidney stones are encouraged to ingest enough water to keep the urine dilute.