Cell-free marginal layer model

In small capillary hemodynamics, the cell-free layer is a near-wall layer of plasma absent of red blood cells since they are subject to migration to the capillary center in Poiseuille flow.^[1] Cell-free marginal layer model is a mathematical model which tries to explain Fåhræus–Lindqvist effect mathematically.

Mathematical modeling

Governing equations

Consider steady flow of blood through a capillary of radius $R$. The capillary cross section can be divided into a core region and cell-free plasma region near the wall. The governing equations for both regions can be given by the following equations:^[2]

${\displaystyle {\frac {-\Delta P}{L}}={\frac {1}{r}}{\frac {d}{dr}}(\mu _{c}r{\frac {du_{c}}{dr}});}$ ${\displaystyle 0\leq r\ \leq R-\delta \,}$

${\displaystyle {\frac {-\Delta P}{L}}={\frac {1}{r}}{\frac {d}{dr}}(\mu _{p}r{\frac {du_{p}}{dr}});}$ ${\displaystyle R-\delta \leq r\ \leq R\ \,}$

where:

$\Delta P$ is the pressure drop across the capillary

$L$ is the length of capillary

${\displaystyle u_{c}}$ is velocity in core region

${\displaystyle u_{p}}$ is velocity of plasma in cell-free region

${\displaystyle \mu _{c}}$ is viscosity in core region

${\displaystyle \mu _{p}}$ is viscosity of plasma in cell-free region

$\delta$ is the cell-free plasma layer thickness

Boundary conditions

The boundary conditions to obtain the solution for the two differential equations presented above are that the velocity gradient is zero in the tube center, no slip occurs at the tube wall and the velocity and the shear stress are continuous at the interface between the two zones. These boundary conditions can be expressed mathematically as:

• ${\displaystyle \left.{\frac {du_{c}}{dr}}\right|_{r=0}=0}$
• ${\displaystyle \left.u_{p}\right|_{r=R}=0}$
• ${\displaystyle \left.u_{p}\right|_{r=R-\delta }=\left.u_{c}\right|_{r=R-\delta }}$
• ${\displaystyle \left.\tau _{p}\right|_{r=R-\delta }=\left.\tau _{c}\right|_{r=R-\delta }}$

Velocity profiles

Integrating governing equations with respect to r and applying the above discussed boundary conditions will result in:

${\displaystyle u_{c}={\frac {\Delta PR^{2}}{4\mu _{p}L}}[1-({\frac {R-\delta }{R}})^{2}-{\frac {\mu _{p}}{\mu _{c}}}({\frac {r}{R}})^{2}+{\frac {\mu _{p}}{\mu _{c}}}({\frac {R-\delta }{R}})^{2}]}$

${\displaystyle u_{p}={\frac {\Delta PR^{2}}{4\mu _{p}L}}[1-({\frac {r}{R}})^{2}]}$

Volumetric flow rate

Total volumetric flow rate is the algebraic sum of the flow rates in core and plasma region. The expression for the total volumetric flow rate can be written as:

${\displaystyle Q={\frac {\pi \Delta PR^{4}}{8\mu _{p}L}}[1-(1-{\frac {\delta }{R}})^{4}(1-{\frac {\mu _{p}}{\mu _{c}}})]}$

Comparison with the viscosity which applies in the Poiseuille flow yields effective viscosity, $\mu _{{e}}$ as:

${\displaystyle \mu _{e}={\frac {\mu _{p}}{[1-(1-{\frac {\delta }{R}})^{4}(1-{\frac {\mu _{p}}{\mu _{c}}})]}}}$

It can be realized when the radius of the blood vessel is very larger than the thickness of the cell-free plasma layer, the effective viscosity is equal to bulk blood viscosity ${\displaystyle \mu _{c}}$ at high shear rates (Newtonian fluid).

See also

• Fåhræus–Lindqvist effect
• Blood viscosity
• hemodynamics

References

1. W. Pan, B. Caswell and G. E. Karniadakis (2010). "A low-dimensional model for the red blood cell". Soft Matter. doi:10.1039/C0SM00183J.
2. Krishnan B. Chandran, Alit P. Yoganathan , Ajit P. Yoganathan , Stanley E. Rittgers (2007). Biofluid mechanics : the human circulation. Boca Raton: CRC/Taylor & Francis. ISBN 978-0-8493-7328-2.