Red blood cells are highly deformable
In this movie red blood cells can be observed flowing in capillaries in skeletal muscle. A closer look reveals that red blood cells are highly deformable - this enables them to traverse microvessels whose diameter is only about 3µm (the capillary on the left). Notice also that in capillaries, red blood cells travel in single file separated by plasma gaps that are sometimes quite large. A consequence of this is that important properties such as blood viscosity or oxygen concentration are far from uniform, and can be highly variable both spatially and temporally. This in turn affects our understanding of flow and exchange of materials in microvessels.