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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.^[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.^[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.

The glycocalyx is the carbohydrate-rich coat at a cell's external surface. On animal cells it includes sugar chains attached to glycoproteins, glycolipids and proteoglycans, together with associated extracellular material. It is outside the lipid bilayer, rather than an extra membrane leaflet. Its exposed molecular structures contribute to protection and interactions between cells.^[ref]

A useful distinction is between the anatomical boundary and the region accessible to a tracer. A surface coat may contain water yet restrict a large molecule, so a dark region in a tracer image need not be an empty gap or part of the cell's interior. A carbohydrate-binding probe instead reports binding sites; these measurements answer different questions about the same coat.

An experiment on hamster muscle capillaries compared vessel boundaries with the regions occupied by blood cells and fluorescent dextran. Bright-field and membrane-label observations located the wall, while the dextran column occupied a narrower region. Sustained light-dye treatment widened the accessible tracer column. The observations supported a surface layer that excluded some circulating material, rather than a simple identity between anatomical lumen and accessible lumen.^[ref]

For an original geometric illustration, a vessel of radius $3\mu \mathrm{m}$ with an inaccessible layer $0.5\mu \mathrm{m}$ thick leaves the tracer an accessible area fraction $(2.5/3{)}^2=25/36$, about $69\%$. This is an illustrative cylindrical model, not a universal measured glycocalyx thickness. Accessibility also depends on the tracer and the state of the surface layer.

Interstitial fluid is the extracellular fluid between the body's cells, outside the circulating blood plasma. It provides the immediate fluid environment through which many dissolved substances move between capillaries and cells. Fluid entering lymphatic vessels becomes lymph, so exchange with blood and drainage through the lymphatic system jointly influence its volume.^[ref]

A simplified volume balance iswhere $J_v$ is net filtration into the interstitial compartment and $J_L$ is lymphatic outflow. In an illustrative compartment, inflow and outflow of $2\mathrm{mL}\mathrm{mi}{\mathrm{n}}^{-1}$ leave volume steady even though fluid continues moving. If inflow becomes $3$ while outflow stays $2$, the compartment gains $1\mathrm{mL}$ each minute. A steady volume therefore does not imply an absence of exchange.

The barrier's microscopic structure also matters. In 2004 experiments, researchers perfused rat microvessels with albumin solution and adjusted albumin around the vessel. A substantial effective oncotic pressure still opposed filtration even when bulk tissue and vessel concentrations matched. Imaging and modeling supported a protected concentration region near the endothelial glycocalyx. The result shows why bulk interstitial fluid composition alone does not fully specify the forces across the vessel wall.^[ref]