Distribution of size and shape in populations of normal human red cells

The diameter, area, and volume of individual human erythrocytes (of 8 subjects, newborn to age 71) were determined by photographing the cells hanging on edge. Measurements from high magnification prints were processed by computer. The distributions of diameter, area, and volume are described statistically, with the unexpectedly linear regression equations for their interrelations. The plot of area vs. volume for the 1016 normal cells from seven subjects (newborn excluded) was remarkably linear with a "straight-line" boundary restricting the distribution. Shape was characterized by a dimensionless "sphericity index" (4.84.volume 2/3 /area). Cells of larger volume tended to be thinner than the smaller cells.

The red cell can easily be deformed at constant volume, but an increase in membrane area results in hemolysis. A theoretical geometric parameter, the "minimum cylindrical diameter" (MCDiam), in microns, the thinnest cylindrical channel through which each individual cell could pass, predicts the linear boundary of the plot of area vs. volume. The MCDiam value of 3.66 µ ± 0.04 SEM accurately represents the thinnest channel through which 95% of the cells can pass.

In two splenectomized patients with hereditary spherocytosis the MCDiam was increased to approximately 4.0 µ, suggesting that the severest restriction is located in the spleen.