Ionic and osmotic equilibria of human red blood cells

A healthy mature human red blood cell (RBC) lacks nucleus and it is a disc-shaped under physiological conditions (Li et al., 2007). The biconcave discocyte RBC has flexible bi-layer membrane with a high surface-to-volume ratio that facilitates large reversible elastic deformation as it repeatedly passes through narrow capillaries during microcirculation which is necessary to transport oxygen and carbon dioxide though haemoglobin molecules which are contained in the RBC intracellular are essential for gas transport within the circulation. The discocyte shape of RBC is encoded in the mechanical properties of its bilayer-membranes: 7.8µm in diameter; 136µm2 in surface area, 85.1µm3 in volume and 1.7-2.2µm in thickness (Tse and Lux, 1999; Turgeon, 2004). Some variations in size, shape or colour of RBC may be seen on microscopic examination with Write's Romanosky – type stain (Dunphy (2010). The movement and distribution of ions across RBC membranes is greatly influenced by the presence of charged impermeable macromolecules which prompt an equivalent number of oppositely charged permeable ions to remain with them in the same compartment in which they occur (Donnan, 1911; Lang, 2007). The main basic function of the sodium pump is to maintain the Na+/K+ gradients across the membrane. Thus, membrane potential, nutrients uptake, intracellular pH and volume are all regulated by the integrity of functional sodium pump. Also, since solutions of extracellular and intracellular are asymmetric in concentration, diffusion of water in one direction exceeds that in the other and the net movement of water across the membrane continues until the concentrations of solute becomes the same on both sides of the membrane or until the force generated by the osmosis is balanced by some opposing pressure resulting from the tendency of one solution to increase in volume at the expense of the other. So, any change in the extracellular pH affects the intracellular solution (Lardner, 2001; Swietach et al., 2010) and the plasma barrier mechanics contributes to the explanation of stomatocyte-discocyte-echnocyte shape bending rigidity.