The Effect of pH on the Volume, Density and Shape of Erythrocytes and Thymic Lymphocytes*

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Abstract
Summary Erythrocytes increase in volume, decrease in density and become more spherical as the pH of isotonic media decreases over the range 9.4–4.8. Swelling is rapid, reversible, occurs in a wide range of media and is independent of gas exchange; there is strong indirect evidence that it is independent of metabolic activity. The swelling is largely the result of entry of anions in the cell as the pH falls, in accordance with the degree of ionization of haemoglobin and Gibbs‐Donnan equilibria. Human erythrocytes show a complex response to the anion uptake, fail to act as perfect osmometers and ‘resist’ swelling in isotonic solutions. Rat erythrocytes differ, and swell to the extent predicted from their anion uptake.By contrast, the volume of thymic lymphocytes does not normally vary with pH. Small changes in volume are observed in acid media containing acetate or phthalate, but no change occurs in albumin solutions used for density gradient analysis.The results emphasize the need for control of pH as well as tonicity when using erythrocytes to calibrate Coulter Counter apertures, or when performing size range or density distribution analyses of erythrocyte populations.

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