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C. Desjardins is a coauthor with B. R. Duling of a 1990 experiment investigating the endothelial glycocalyx and red blood cell distribution in capillaries. They perfused individual capillaries with heparinase and measured tube hematocrit, the fraction of the local vessel volume occupied by red blood cells. The measured fraction increased at least twofold without a significant change in red blood cell velocity.^[ref]

The controls make the inference useful. Vehicle alone and heat-denatured heparinase did not produce the same effect, arguing against simple perfusion or the mere presence of added protein as explanations. Measuring both tube hematocrit and velocity also distinguishes more red blood cells occupying a segment from faster movement through it. The results supported a role for components of the inner capillary surface in cell distribution, without directly measuring every structural feature of the endothelial glycocalyx.^[ref]

Tube hematocrit is the fraction of a blood vessel segment's internal volume occupied by red blood cells at a given time:It is a local occupancy measurement. It can differ from the hematocrit of a collected blood sample or the fraction of transported blood volume carried by red cells per unit time.^[ref]

An idealized capillary segment containing $120\mathrm{pL}$ of red blood cells in a total $400\mathrm{pL}$ has $H_T=0.30$. Observing more cells in that segment can raise $H_T$ without creating new cells. The sampled vessel length, cell positions and occupied volume matter.

The contrast with a flux measurement follows from conservation. If red cells in an idealized steady segment have mean axial speed $u_R$ and the volume-averaged blood speed is $u_B$, then red-cell volume flux is $H_{T}A{u}_R$ and total blood volume flux is $A{u}_B$. Their ratio is $H_D=H_T{u}_R/u_B$. If $u_R>u_B$, the transported red-cell fraction $H_D$ exceeds local occupancy $H_T$. This simplified relation assumes the indicated averages describe the same segment and flow.

C. Desjardins and B. R. Duling used in-situ microperfusion to perturb the endothelial glycocalyx. Heparinase increased capillary tube hematocrit at least twofold without a significant red-cell-velocity change; vehicle and heat-denatured enzyme did not reproduce the effect. This supports a contribution from the vessel's surface environment to red-cell occupancy, rather than interpreting occupancy solely as a speed measurement.^[ref]