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."

Nephrotic syndrome

Nephrotic syndrome is a set of signs or symptoms that may point to kidney problems. The kidneys are two bean-shaped organs found in the lower back. Each is about the size of a fist. They clean the blood by filtering out excess water and salt and waste products from food. Healthy kidneys keep protein in the blood, which helps the blood soak up water from tissues. But kidneys with damaged filters may leak protein into the urine. As a result, not enough protein is left in the blood to soak up the water. The water then moves from the blood into body tissues and causes swelling.