Blood contains proteins that help build bones and muscles, fight infection, and balance fluids in the body. The main protein in blood is albumin. Your kidneys remove extra waste and fluid from your blood. Healthy kidneys allow protein and other nutrients in the blood to pass through and remain in the blood stream. When kidneys suffer damage, some of the protein molecules escape into the urine. Doctors refer to protein in the urine as proteinuria or albuminuria. It’s a sign that there’s something wrong with your kidneys.