= Oxyhemoglobin
{title2=$\mathrm{O_2Hb}$}
\b[<Oxyhemoglobin>] is oxygen-bound <hemoglobin>. Binding is reversible: an available <haem> site can acquire or release an oxygen molecule as conditions change. A fully occupied adult hemoglobin tetramer carries four oxygen molecules, one per haem site.<Hemoglobin by Radiometer>[Structure and function; additional Oxygen saturation parameter page, Physiological background]{ref} Oxygenation describes ligand occupancy and should not be confused with the ferric haem state defining <methemoglobin>.
A quantitative description must specify its denominator. The manufacturer's functional <oxygen saturation> is
$$
s_{\mathrm O_2}=\frac{c_{\mathrm O_2\mathrm{Hb}}}{c_{\mathrm O_2\mathrm{Hb}}+c_{\mathrm{HHb}}},
$$
where HHb denotes unoxygenated, oxygen-capable hemoglobin. A fractional oxyhemoglobin quantity instead divides by total hemoglobin, including specified non-oxygen-carrying derivatives.<Hemoglobin by Radiometer>[Additional Oxygen saturation parameter page, definition of functional hemoglobin; total-hemoglobin definition]{ref} The concentrations must use a consistent haem-equivalent convention.
For an original four-component example, suppose those equivalent concentrations are 80 units <oxyhemoglobin>, 5 units <deoxyhemoglobin>, 10 units <carboxyhemoglobin> and 5 units <methemoglobin>. Then functional saturation is $80/85\approx94.1\%$, whereas the oxyhemoglobin fraction of the full 100-unit total is $80\%$. These describe the same hypothetical mixture with different denominators. Neither percentage alone states the total oxygen amount in a blood volume.
A separate amount calculation assumes 2 micromoles of tetramers, all with four functional sites, at mean occupancy $0.75$. Bound oxygen is then
$$
n_{\mathrm O_2}=2\times4\times0.75=6\,\mathrm{\mu mol}.
$$
Any oxygen dissolved outside hemoglobin is an additional pool. Concentration, binding fraction and gas <partial pressure> are related through a physical model, rather than being interchangeable measurements.
Primary spectral work measured <oxyhemoglobin> alongside other derivatives, providing coefficient data for distinguishing their contributions to <absorbance>.<Absorption spectra of human fetal and adult oxyhemoglobin, de-oxyhemoglobin, carboxyhemoglobin, and methemoglobin by W. G. Zijlstra and others>[Abstract, 450–1000 nm derivative spectra]{ref} This connects molecular oxygenation with a measurable optical signal while retaining the distinction between one derivative and the whole sample.
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