Question

why protein is not normally found in urine

Answer 1

The kidney is responsible for reabsorption - ions and water molecules can cross the membrane and hence there is an osmotic gradient for proteins to be reabsorbed along with them, so they don't end up the urine. Proteins in the urine is usually due to a problem: excessive drinking (more than 4l of water per day), damage to nephrons or preeclampsia can all cause proteinurea (protein in the urine).

Answer 2

thanks for your help :D.