Red blood cell ion transport mechanisms

Schematic representation of the transport mechanisms regulating red cell hydration. The extracellular concentrations of sodium and calcium are higher than those within the cell, creating favorable gradients for entry, while the intracellular concentration of potassium is higher than that in the extracellular fluid, creating a favorable gradient for potassium exit by the K-Cl cotransporter or the calcium-activated (Gardos) potassium channel. The red transporters are active, the blue transporters are passive. Band 3 protein primarily functions as a Cl-HCO3 exchanger. Its primary physiological function is to facilitate CO2 transport from tissues to alveoli; it also plays an important role in defining red cell shape and membrane stability. Water movement passively follows that of cations and anions, or changes in tonicity of the red cell's environment. Transport of water can occur at a much faster rate via water channels (aquaporin-1, Aqp-1).