Adjacent to the walls of our arterioles（小动脉）, capillaries¹（毛细血管）, and venules（小静脉） -- the blood vessels³ that make up our microcirculation -- there exists a peculiar⁴ thin layer of clear plasma⁵, devoid⁶ of（没有，缺乏） red blood cells. Although it is just a few millionths of a meter thick, that layer is vital. It controls, for example, the speed with which platelets can reach the site of a cut and start the clotting⁷ process. "If you destroy this layer, your bleeding time can go way up, by 60 percent or more, which is a real issue in trauma⁸," said Eric Shaqfeh, the Lester Levi Carter Professor and a professor of chemical engineering and mechanical engineering at Stanford University. Along with his colleagues, Shaqfeh has now created the first simplified computer model of the process that forms that layer -- a model that could help to improve the design of artificial platelets and medical treatments for trauma injuries and for blood disorders⁹ such as sickle¹⁰ cell anemia¹¹ and malaria¹².

 

The model is described in a paper appearing in the journal Physics of Fluids.

 

The thin plasma layer, known as the Fåhræus-Lindqvist layer, is created naturally when blood flows through small vessels. In the microcirculation, the layer forms because red blood cells tend to naturally deform¹³ and lift away from the vessel² walls. "The reason they don't just continually move away from the wall and go far away is because, as they move away, then also collide with other red blood cells, which force them back," Shaqfeh explained. "So the Fåhræus-Lindqvist layer represents a balance between this lift force and collisional forces that exist in the blood."

 

Because the deformation¹⁴ of red blood cells is a key factor in the Fåhræus-Lindqvist layer, its properties are altered in diseases, such as sickle cell anemia, that affect the shape and rigidity¹⁵ of those cells. The new model, which is a scaled-down version of an earlier numerical model by Shaqfeh and colleagues that provided the first large-scale, quantitative¹⁶ explanation of the formation of the layer, can predict how blood cells with varying shapes, sizes, and properties -- including the crescent-shaped cells that are the hallmark of sickle cell anemia -- will influence blood flow.

 

The model can also help predict the outcome of -- and perfect -- treatments for trauma-related injuries. One common thing to do during treatment for trauma injuries is to inject saline, which among other things reduces the hematocrit, the blood fraction of red blood cells. With our model, Shaqfeh said, "we can predict how thick the Fåhræus-Lindqvist layer will be with a given hematocrit, and therefore how close the platelets will be to the periphery¹⁷（外围，边缘） of the blood vessels -- and how quickly clotting will occur.

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