Cells of the excretory tubule remove water by osmosis, from which we can conclude that the solute concentration in these cells must be lower than in the filtrate. The excretory tubule remove excess amount of water from the cell through semi permeable membrane of the cell while on the other hand, the solute concentration in the cell is lower as compared to filtrate so that's why it can't diffuse from the cell by the process of osmosis.
