The marginal plasma layer devoid of red cells, often observed in arteriolar blood flow, is discussed in terms of a simplified continuum model. The plasma is assumed to flow through porous medium and to be described by the Darcy-Stokes equation. In order to relate the permeability of the porous medium to the hematocrit, a drag formula suggested by Brinkman and Tam is utilized. Further, a rough estimation for the thickness of plasma layer is made on the basis of the shape of velocity profiles thus obtained. It is shown that the result is in substantial agreement with experiment by Charm et al.