In blood circulation, red blood cells tend to migrate away from the vessel wall toward the centerline, leaving a cell-depleted region near the wall, known as the cell-free layer. The composition of plasma is extremely important from a physiological point of view, but regarding fluid mechanics, it is considered a Newtonian fluid with fluid properties comparable to water. Newton's second law of motion can be applied to fluids: the sum of forces acting on a fluid is equal to the product of its mass by acceleration. This principle is called the conservation of momentum. Viscous stresses oppose relative movements between neighboring fluid particles and can be described by tangential shear and normal components. Nevertheless, it is sometimes appropriate to use the equations developed in straight geometries for tortuous geometries, as long as inertial forces are negligible as compared with the other forces present, such as the friction forces.