Estimation of viscous dissipation inside an erythrocyte during aspirational entry into a micropipette.

Viscous dissipation inside the erythrocyte during its aspirational entry into a micropipette is analyzed. The motion of the intracellular fluid is approximated by a flow into the micropipette orifice from a half space (the portion of the erythrocyte outside the micropipette). The stream function and intracellular pressure (p) in the half space are obtained as a function of radial and axial positions near the orifice. Solution of the boundary value problem for a uniform stream entering a circular hole gives p = 2 eta HQ/pi R3p, where eta H is the intracellular viscosity, Q is the total discharge, and Rp is the pipette radius. The results indicate that the moving erythrocyte membrane helps to drive the intracellular fluid into the orifice. For normal erythrocytes, p is only approximately 0.5% of the total aspiration pressure (delta P). The contribution of p to delta P, however, may become significant when there is a large increase in eta H due to a markedly elevated intracellular hemoglobin concentration or an alteration of the physical state of hemoglobin.

Flow of particles along a deformable tube.

Slow viscous flow of rigid particles along a deformable tube of comparable diameter is considered as a possible model for some biological flows. Lubrication theory is assumed to be valid in the fluid region. The cylindrical tube is considered to be a thin elastic shell undergoing small deflections. The mean velocity of the flow is assumed to be maintained at a constant value by the application of a pressure difference over some length including the particle, or by an external force acting directly on the particle. Numerical results are obtained for the force required to maintain the motion and for the distribution of fluid pressure and thickness along the tube as a function of the diameter ratio, dimensionless velocity parameter and the shape of the particle. Effect of the bending resistance of the tube on the flow is also discussed.

Passive mechanical properties of human leukocytes.

Micropipette experiments are used to determine the rheological properties of human leukocytes. Individual cells in EDTA are subjected to a known aspiration pressure via a micropipette, and their surface deformation from the undeformed spherical shape is recorded on a television monitor. The cells are mathematically modeled as homogeneous spheres, and a standard solid viscoelastic model is found to describe accurately the deformation of the cell for small strains. These experimental and theoretical studies provide the basis for further investigations of leukocyte rheology in health and disease.