Kidney

"The kidneys are a pair of organs located in the back of the abdomen. Each kidney is about 4 or 5 inches long -- about the size of a fist.
The kidneys' function are to filter the blood. All the blood in our bodies passes through the kidneys several times a day. The kidneys remove wastes, control the body's fluid balance, and regulate the balance of electrolytes. As the kidneys filter blood, they create urine, which collects in the kidneys' pelvis -- funnel-shaped structures that drain down tubes called ureters to the bladder.
Each kidney contains around a million units called nephrons, each of which is a microscopic filter for blood. It's possible to lose as much as 90% of kidney function without experiencing any symptoms or problems."

Renal colic

Renal colic is a type of pain caused by kidney stones. Kidney stones are crystals that form from chemicals in the urine. Usually, a stone develops because too much of a single chemical is present in the urine. A stone may block the flow of urine and can cause pain if it travels down the tubes of the urinary tract. Symptoms of renal colic are related to the formation of crystals in the kidneys. These stones can block urine output, causing pain, swelling and infection.