Urinary system video and assignment, what is a kidney stone and what causes it?

A kidney stone is a solid piece of material that forms in a kidney when substances that are normally found in the urine become highly concentrated. A stone may stay in the kidney or travel down the urinary tract. Kidney stones vary in size. A small stone may pass on its own, causing little or no pain. A larger stone may get stuck along the urinary tract and can block the flow of urine, causing severe pain or bleeding.Kidney stones are one of the most common disorders of the urinary tract.

What causes it?

Kidney stones can form when substances in the urine—such as calcium, oxalate, and phosphorus become highly concentrated. Certain foods may promote stone formation in people who are susceptible, but scientists do not believe that eating any specific food causes stones to form in people who are not susceptible. People who do not drink enough fluids may also be at higher risk, as their urine is more concentrated. Usually people who don't drink enough water. It is known that kidney stones occur far more often in men then in women.

http://www.youtube.com/watch?v=LngbrHJkXoE