How is selective reabsorption acheived in the kidneys?

In the cells lining the proximal convoluted tubule, sodium-potassium pumps present in the membrane in contact with tissue fluid pump sodium ions out of the cells. This lowers the concentration of sodium ions in the cell cytoplasm thereby causing sodium ions to enter the cells, accompanied by a glucose molecule or an amino acid, from the lumen down a concentration gradient by facilitated diffusion. The concentration of amino acids and glucose inside the cell therefore increases and they then diffuse down a concentration gradient into the tissue fluid. Water follows the path of these molecules as it moves down the water potential gradient. These substances then diffuse into the blood from the tissue fluid and are carried away to where they are needed.