Effects of shape and size on red blood cell deformability: a static bending analysis.

When flowing down a tapered tube, such as a narrow capillary, red blood cells (RBCs) are subject to deformation, the first event of which is folding in a pancake manner. The RBC deformability is reduced during cell ageing, a phenomenon that may reflect alterations in intracellular viscosity, membrane rigidity or RBC shape. Age related shape changes and their importance for increased RBC rigidity were theoretically analysed. The average empirically observed RBC profile is shown to offer little resistance to bending as compared to other, theoretically possible profiles of the same membrane area and RBC volume. Because of a decrease in projected area (diameter size), and therefore in pressure load, the pressure needed to initiate folding of an old RBC is between 20 and 55% higher than that required to fold a young one if, during RBC ageing, membrane area to cell volume ratio is constant as empirically observed. This difference exists whether the RBC is mathematically treated as a solid body or as a membrane shell.