Kidney stones are solid accumulations of material that form in the tubal system of the kidney. They cause painful problems when they block the flow of urine through or out of the kidney.

Urine is formed by the kidneys. As blood flows into the kidneys, specialized tubes within the kidneys extract a certain amount of fluid and other substances from the blood, which then flows out of the body as urine. Sometimes, a problem in the kidneys makes these dissolved substances become solid again. Sometimes, tiny crystals called "silent stones" may form in the urine that never cause any pain.

Kidney stones cause problems when they interfere with the normal flow of urine. They can block the flow down the ureter--the tube that carries urine from the kidney to the bladder. Because the kidney isn't used to feeling any pressure, when pressure builds from backed-up urine...