The process

• (1) Firstly sodium ions are actively transported out of the cells lining the proximal convoluted tubule into blood capillaries which carry them away. The sodium ion concentration of these cells is therefore lowered.
• (2) Sodium ions now diffuse down a concentration gradient from the lumen of the proximal convoluted tubule into the epithelial lining cells, but only through special carrier proteins by facilitated diffusion.
• (3) These carrier proteins are of specific types, each of which carries another molecule (glucose/amino acids/chloride ions) along with the sodium ions. This is known as co-transport.
• (4) The molecules which have been co-transported into the cells of the proximal convoluted tubule then diffuse into the blood. As a result, all the glucose and most other valuable molecules are reabsorbed as well as the water.