Band 3 protein is a typical polytropic memberane protein that mediates the exchange of cellular bicarbonate ions $\left(\mathrm{HCO}_3^{-}\right)$with the chloride $\left(\mathrm{Cl}^{-}\right)$ion in plasma, which has been known as the 'chloride shift.' Owing to this, RBC can discriminate the metabolically active cells from inactive cells and deliver oxygen particularly to metabolically active tissue that produce carbon dioxide.