= Hematocrit
\b[Hematocrit] is the fraction of a blood sample's <volume> occupied by <red blood cells>. It can be expressed as a dimensionless fraction or as a percentage:
$$
H=\frac{V_{\mathrm{red\ cells}}}{V_{\mathrm{blood}}},\qquad H_{\%}=100H.
$$
For example, $0.45\,\mathrm{mL}$ of <red blood cells> in $1.00\,\mathrm{mL}$ of blood corresponds to $H=0.45$, or 45%. It is a volume fraction, rather than the number of cells per unit <volume> or the concentration of <hemoglobin>.
The denominator matters. In an idealized sample, adding $0.50\,\mathrm{mL}$ of cell-free plasma to that mixture without changing its red-cell <volume> reduces <hematocrit> to $0.45/1.50=0.30$. Thus a change in <hematocrit> does not by itself establish a change in the total number of <red blood cells>. Interpretation depends on both cell volume and the surrounding fluid.
Local measurements also need a specified sampling context. Capillary tube <hematocrit> is the instantaneous fraction of a small vessel's internal <volume> occupied by <red blood cells>; it need not equal the value in a collected systemic sample. <C. Desjardins> and <B. R. Duling> perfused capillaries with <heparinase> and observed at least a twofold rise in tube <hematocrit> without a significant change in red-cell velocity. Vehicle and heat-denatured-enzyme controls did not reproduce the response.<Heparinase treatment suggests a role for the endothelial cell glycocalyx in regulation of capillary hematocrit by C. Desjardins and B. R. Duling>[Abstract: definition of tube hematocrit, in-situ enzyme microperfusion, controls and velocity measurements.]{ref}
That experiment links local blood-cell distribution to the <endothelial glycocalyx>. A local increase can therefore reflect how cells occupy a vessel, without requiring production of new cells during the observation.
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