Fåhræus effect

Fahraeus effect

Fåhræus effect refers to the phenomenon observed in the microcirculation of blood, where the red blood cells (RBCs) tend to migrate towards the center of a blood vessel, leaving a cell-free layer near the walls. This effect, first described by the Swedish pathologist Robin Fåhræus in 1929, has significant implications for the rheology (flow properties) of blood, particularly in small vessels (capillaries and arterioles) with diameters less than 300-400 micrometers.

Mechanism

The Fåhræus effect is primarily a consequence of the physical interactions between the flowing blood and the vessel walls. As blood moves through a narrow vessel, the RBCs experience less resistance in the center of the vessel compared to near the walls, due to the no-slip condition at the wall where the fluid velocity is zero. This differential in velocity causes RBCs to migrate towards the vessel center, resulting in a reduction of the effective viscosity of blood in these small vessels. The cell-free layer that forms near the vessel walls further reduces friction, allowing blood to flow more easily.

Physiological Significance

The Fåhræus effect plays a crucial role in the efficient oxygenation of tissues and the regulation of blood flow in the microcirculation. By reducing blood viscosity in smaller vessels, it helps to maintain a higher rate of blood flow with less energy expenditure by the heart. This effect also contributes to the non-Newtonian behavior of blood, meaning that its viscosity changes in response to varying shear rates, which is essential for the adaptive response of the circulatory system to changing physiological demands.

Related Phenomena

The Fåhræus effect is closely related to the Fåhræus-Lindqvist effect, which describes the decrease in blood viscosity with decreasing vessel diameter beyond what would be expected from classical fluid dynamics. Together, these effects highlight the complex nature of blood flow in the microcirculation and the importance of considering the unique properties of blood as a suspension of cells, rather than a simple fluid.

Clinical Implications

Understanding the Fåhræus effect is important in various clinical contexts, including the development of artificial blood substitutes, the design of microfluidic devices for lab-on-a-chip applications, and the management of diseases affecting microcirculation, such as diabetes mellitus and sickle cell anemia. It also has implications for the interpretation of hematocrit measurements, as the local hematocrit in small vessels can be significantly lower than systemic hematocrit levels due to this effect.

Research and Applications

Research into the Fåhræus effect and its implications continues to be an active area of study in both the basic and applied sciences. Advances in microfluidics and high-resolution imaging techniques have provided new tools for studying blood flow dynamics at the microscale, with potential applications in drug delivery, diagnostics, and the development of new treatments for circulatory disorders.