6. Perfused boundary region (PBR)of the sublingual arterial microvessels (ranged from 5-25 microns) using Sideview Darkfield imaging. (Microscan, Glycocheck) .The PBR in microvessels is the cell-poor layer which results from the phase separation between the flowing red blood cells (RBC) and plasma.The PBR includes the most luminal part of glycocalyx that does allow cell penetration. Increased PBR is considered an accurate index of reduced endothelial glycocalyx thickness because of a deeper RBC penetration in the glycocalyx.