Viscosity and density measurements on the cytosol of human red blood cells

We introduce an approach for determining the viscosity of the intracellular liquid, called cytosol, of human red blood cells (RBCs). This methodology combines measurements of the mass density distribution of RBCs and the viscosity of the cytosol relative to its water content. The density distribution is obtained through buoyant density centrifugation paired with cell counting. By correlating the Gaussian distribution of cell population densities with the viscosity-density relationship of the cytosol, we derive a log-normal distribution of the cytosol viscosity in healthy RBCs. The viscosity contrast λ=η/η_plasma, which is the ratio between viscosities of the RBC cytosol and blood plasma under physiological conditions, is found to have a mean value of λ¯=10. This value is notably higher than those cited in existing literature for numerical simulations. The broad range of viscosity values stems from the gradual loss of water from RBCs over their 120-day lifespan. Our findings indicate that older RBCs exhibit more than twice the cytosol viscosity of younger cells, a critical factor for future theoretical studies of physiological conditions.