The marginal flow of the blood is the blood flow in close proximity to the walls of the vessels. Especially in the smaller vessels, it is a plasmatic marginal current without leukocytes and erythrocytes, which has a much lower flow velocity than the central blood current. During inflammatory reactions, the marginal flow changes.
What is the marginal current?
The marginal flow of the blood is the flow of blood in close proximity to the walls of the vessels. By the marginal flow of the blood, medicine understands a phenomenon within the Fåhraeus-Lindqvist effect. This effect is a basis of blood flow, which is based on the fluidity of red blood cells and affects the viscosity of blood. In the vessels of the periphery, because of the Fåhraeus-Lindqvist effect, viscosity is much lower within vessels with low lumen than in vessels with higher lumen. Near the vessel walls, shear forces act on the red blood cells. The resulting shear forces lead to displacement of the red blood cells and cause the red blood cells to undergo what is known as axial migration, which gives rise to an axial flow. Simultaneously with the axial migration of the red blood cells, marginal currents with few cells are formed near the vessel walls. The edge currents of plasma wash around the cells and act as a kind of sliding layer for the blood cells in the Fåhraeus-Lindquist effect. In larger vessels, the plasmatic boundary current is negligible because it occupies only a small fraction of the vessel cross-section. Only in pre-capillary and post-capillary vessels with a small cross-section does it account for a significant portion.
Function and role
Marginal flow of blood is observed in all vessels because shear forces are effective in areas close to the vessel walls. However, from a medical point of view, the marginal flow in vessels with a larger lumen is not as relevant as in vessels with a smaller cross-section. In small cross-sections, the shear forces acting on the walls cause the individual components of the blood to redistribute. In this context, the blood is to be regarded as a suspension whose largest particles migrate into the faster-flowing axial flow due to the shear forces. Leukocytes are the largest components of the blood. They are located in the immediate center of the axial flow after migration. Somewhat more peripherally from this, the erythrocytes move on. Even further in the periphery, platelets move. Thus, in vessels of small diameter, a marginal flow of pure plasma, containing hardly any blood cells, is formed during normal blood flow. Blood flow is governed by laws of hemodynamics. These include Darcy’s law and Hagen-Poiseuille’s law. For this reason, the flow behavior of blood depends primarily on blood pressure, vascular resistance and blood viscosity. Blood is an inhomogeneous suspension of blood plasma and blood cells. Blood viscosity does not follow any constancy, but depends on the flow velocity and increases with slow blood flow. Especially the erythrocytes of the blood tend to aggregate at low shear rates. As soon as the blood reaches a faster flow rate, the aggregates break up. In this way, a non-proportional, erratic flow behavior develops, which makes the blood a non-Newtonian fluid. This relationship is relevant only in the smaller vessels. In larger vessels, blood behaves approximately as a Newtonian fluid. The peripheral flow of blood always lags behind the central flow in its velocity. Sometimes, blood is also referred to as having a double flow behavior, which is composed of the near-wall boundary flow and the central flow. The composition of the peripheral flow and the central flow differs depending on the vessel diameter. Basically, platelets tend to move in the marginal flow, whereas leukocytes tend to move in the central flow.
Diseases and disorders
Under pathologic conditions, however, leukocytes may preferentially move in the marginal flow of the blood. This phenomenon accounts, for example, for the so-called sludge phenomenon. In the sludge phenomenon, the erythrocytes of the blood accumulate in the context of microcirculatory disturbances. One consequence of this erythrocyte aggregation is a slower flow velocity and a subsequent reduced oxygen supply to affected tissues.A microcirculatory disorder is any type of restricted blood flow within the smallest blood vessels. Microcirculatory disorders impair not only the supply of oxygen but also the supply of nutrients to the tissues. The disorders are caused by restricted blood flow or impaired exchange of substances within blood vessels less than 100 µm in diameter. In addition to the rheological properties of blood, microcirculation is primarily dependent on blood pressure and ultimately on vessel diameter. However, these factors are susceptible to disruption. When there is insufficient outflow in the venous system, blood backs up in the capillary bed and blood flow is disrupted. In this way, microcirculatory disorders occur with an unusual flow distribution of blood cells. Diseases or pathological phenomena with symptomatic microcirculatory disturbances can be, for example, acute inflammatory reactions. In addition, the circulation disorders occur in the context of pAVK (Peripheral Arterial Occlusive Disease), CHD (Coronary Artery Disease) and the tropical ulcer with insufficiency of the veins. The same is true for gangrene. When there are many leukocytes in the marginal blood stream and the blood flow velocity has decreased, the leukocytes from the marginal stream adhere to the vessel walls. However, this adhesion is reversible. As soon as the flow velocity increases again, the leukocytes are detached from the walls of the vessels and are washed on. Altered marginal flow of blood may also be the result from arteriosclerotic changes within the vessels. In arteriosclerosis, the vessels calcify. Various components deposit on the walls of the vessels, increasingly narrowing the lumen of the affected veins.
