 Tube hematocrit

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]