It also reabsorbs those materials the body still needs which got past the original filtering system. However, there are certain things in the blood which do not get past the filtering system of the kidney simply because they are too big. The red and white blood cells are a good example. This is good thing because we would otherwise need a blood transfusion every time we went to the bathroom. Proteins are large molecules which help make up our muscles, important parts of our immune system, and many other portions of our bodies.
Most proteins are also too large to pass through the filtering system of the kidney.
And since they are not supposed to pass into the kidney, there is no mechanism for proteins to be reabsorbed if they make it in there. Infections, diseases that only involve these microscopic filters, and diseases which affect the kidney as a whole are all examples of processes that might cause protein in the urine. However, a child who is otherwise growing well, not losing a very large amount of protein in the urine, and has a normal blood pressure, the most common cause of protein in the urine is called benign orthostatic proteinuria. Simply put, it means protein shows up in the urine whenever the child is standing. It causes no harm except to strike fear in concern of the parents of the children who have it.