Nephrotic syndrome results from damage to the kidneys' glomeruli—tiny blood vessels that filter wastes and excess water from the blood and send them to the bladder as urine.
When the glomeruli are working properly, they keep protein in the blood from leaking into the urine. Healthy kidneys allow less than 1 gram of protein to escape through the urine in a day. In nephrotic syndrome, the damaged glomeruli allow 3 grams or more of protein to leak into the urine during a 24-hour period.
As a result of this protein loss, the blood is deficient. Normal amounts of blood protein are needed to help regulate fluid throughout the body. Protein acts like a sponge to soak up fluid into the bloodstream. When blood is low in protein, fluid accumulates in the body’s tissues rather than circulating. The fluid causes swelling and puffiness.
Nephrotic syndrome can occur with many diseases. In adults, the most common causes are diabetic nephropathy and membranous nephropathy. In older adults, the most common cause is amyloidosis. Prevention of nephrotic syndrome relies on controlling these diseases. Frequently, however, the cause of nephrotic syndrome is unknown.