A simulation study of the anomalous osmotic behavior of red cells

The factors responsible for movements of water across cell membranes were described mathematically and incorporated into a model which simulates water balance in the cell. Included in the model are a variable charge and osmotic coefficient of hemoglobin, a Na/K pump whose rate varies with ionic concentrations, and the standard electroneutrality and osmotic equilibrium assumptions. The model was used to investigate the phenomena whereby human red cells placed in media of varying tonicities exhibit steady state volume changes less than those predicted by van't Hoff's Law. The model results showed that this anomalous osmotic behavior was primarily due to changes in the osmotic coefficient of hemoglobin as its concentration in the cell varied. A second factor accounting for a part of this behavior was the alteration in the rate of the Na/K pump due to intracellular ionic concentration changes as cell volume varied. The effect of variable electrical charge on the hemoglobin molecule was found to be in the wrong direction to account for the observed osmotic behavior. Also, this effect was seen to produce relatively large changes in cell membrane potential, a result inconsistent with experimental data. It was concluded from the model results that the anomalous osmotic behavior of human red cells is primarily due to the variation in the osmotic coefficient of hemoglobin as the cell volume changes, and that the variable charge effect on the hemoglobin molecule, if it exists, does not play a role in this response.