= Tube hematocrit
\b[Tube hematocrit] is the fraction of a blood vessel segment's internal <volume> occupied by <red blood cells> at a given time:
$$
H_T=\frac{V_{\mathrm{red\ cells\ in\ segment}}}{V_{\mathrm{segment}}}.
$$
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.<Heparinase treatment suggests a role for the endothelial cell glycocalyx in regulation of capillary hematocrit by C. Desjardins and B. R. Duling>[Abstract: instantaneous fractional red-cell volume as capillary tube hematocrit.]{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 $H_D=H_Tu_R/u_B$. 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.<Heparinase treatment suggests a role for the endothelial cell glycocalyx in regulation of capillary hematocrit by C. Desjardins and B. R. Duling>[Abstract: microperfusion, controls and simultaneous velocity comparison.]{ref}
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