Intracellular viscosity of healthy red blood cells is double as high as assumed so far

We present a novel approach to quantifying the intracellular viscosity of human erythrocytes (red blood cells), a critical parameter for accurately predicting cardiovascular diseases and simulating blood flow at the cellular level. While existing models predominantly focus on the viscoelastic properties of the erythrocyte membrane, our study shifts the spotlight to the less understood viscosity of the cytosol—the near-saturated haemoglobin solution within red blood cells. By combining buoyant density centrifugation with cell counting techniques, we precisely measure the mass density distribution of erythrocytes. This density data, when correlated with the viscosity-density relationship of the cytosol, reveals a log-normal distribution of cytosol viscosity in healthy red blood cells. Our findings indicate a significantly higher mean viscosity contrast $\(\bar\lambda=10\)$ between the cytosol and blood plasma than commonly reported in literature. This contrast is attributed to the gradual dehydration of erythrocytes over their 120-day lifespan, resulting in older cells displaying over twice the viscosity of younger ones. These insights not only refine our understanding of blood rheology but also lay the groundwork for more accurate numerical models of blood flow under both healthy and pathological conditions.