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."

Bartter syndrome

Bartter syndrome refers to a group of rare conditions that affect the kidneys. There are five gene defects known to be associated with Bartter syndrome. The condition is present from before birth (congenital). This disease usually occurs in childhood. Symptoms include:
1. Constipation
2. Growth failure
3. Increased frequency of urination
4. Low blood pressure
5. Kidney stone
6. Muscle cramping and weakness.