= Endothelial glycocalyx
The <endothelial glycocalyx> is the carbohydrate-rich surface layer on the blood-facing side of <endothelial cells>. It includes membrane-associated <glycoproteins> and <proteoglycans> together with associated material from plasma. It is a specialization of the <glycocalyx>, not a second layer of endothelial cells. Its interactions with flowing <blood> affect which regions near the vessel wall are accessible to cells and dissolved macromolecules.<Identification of distinct luminal domains for macromolecules, erythrocytes, and leukocytes within mammalian capillaries by H. Vink and B. R. Duling>[Abstract: endothelial surface coat and distinct luminal domains]{ref}
A perturbation experiment tested whether this surface mattered for blood-cell distribution. Investigators microperfused skeletal-muscle <capillaries> with <heparinase> and measured the fraction of capillary volume occupied by <red blood cells>. This tube <hematocrit> increased at least twofold without a significant change in red-cell velocity. Vehicle alone and heat-denatured enzyme did not produce the same effect.<Heparinase treatment suggests a role for the endothelial cell glycocalyx in regulation of capillary hematocrit by C. Desjardins and B. R. Duling>[Abstract: microperfusion, hematocrit, velocity and controls]{ref}
The control matters: exposure to a liquid or protein preparation alone is different from the action of an active <enzyme>. The findings support a functional contribution of the endothelial surface, while not establishing that every aspect of the layer has been measured or removed.
For an original interpretation example, a capillary changing from $12\%$ to $24\%$ red-cell volume fraction has doubled its local tube <hematocrit>. That observation alone does not say that red cells move twice as fast or that total volume flow has doubled. Occupied volume, velocity and flux are related measurements with different definitions.
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