This condition is not due to infection or cancer of the kidney and cannot be spread to other people. Both kidneys are always involved. The glomeruli are the filters of the kidneys which filter the blood and make urine. Normally, large molecules such as proteins and cells such as red blood cells or white blood cells do not pass through the filters andare retained within the blood because they are so important for health.
Small molecules pass completely through the filters.
Other molecules which are not required for body functions are passed freely into the urine, for example urea, uric acid and creatinine. There are two main effects. Firstly, substances which are normally retained in the circulation escape into the urine through the filtration mechanism. As a consequence, protein and red cells appear in the urine and can be detected by a dip stick urine test. Protein in the urine is called proteinuria and blood in the urine is haematuria.