= Arterial oxygen content
{title2=$C_a$}
\b[Arterial oxygen content] is the amount of oxygen carried per specified volume of arterial <blood>. It includes oxygen bound to <hemoglobin> and oxygen dissolved in the liquid phase. It differs from oxygen <partial pressure> and from the fractional occupancy represented by <oxygen saturation>.<Anatomy and Physiology 2e by J. Gordon Betts and others>[Section 22.5, Oxygen Transport]{ref}
A useful quantity model is
$$
C_a=\beta H F_{\mathrm O_2Hb,a}+\alpha P_{a\mathrm O_2}.
$$
Here $H$ is total hemoglobin concentration, $F_{\mathrm O_2Hb,a}$ the <fractional oxyhemoglobin>, $\beta$ a specified oxygen-binding conversion factor and $\alpha$ the dissolved-oxygen coefficient for the stated conditions. Units must be compatible: for example, $H$ in grams per decilitre and $\beta$ in millilitres oxygen per gram give a bound contribution in millilitres oxygen per decilitre blood.
Using the manufacturer's worked coefficient $\beta=1.34\,\mathrm{mL\ O_2\,g^{-1}}$, a chosen $H=15\,\mathrm{g\,dL^{-1}}$ and fraction $0.98$ give bound oxygen about $19.70\,\mathrm{mL\,dL^{-1}}$. Halving $H$ gives about $9.85$ despite unchanged fraction. The dissolved contribution must still be added for total content.<Hemoglobin by Radiometer>[Hemoglobin: Structure and function, binding coefficient and bound/dissolved distinction]{ref} These are amount calculations, not clinical target values.
The saturation definition matters. Total hemoglobin can include <carboxyhemoglobin> and <methemoglobin>, so a fraction defined using only oxygenated plus deoxygenated functional hemoglobin should not automatically be multiplied by total $H$ as though every included form had the same oxygen-binding status.<Hemoglobin by Radiometer>[Total-hemoglobin definition and dysfunctional species]{ref}
A primary 1989 instrument illustrates why carrier amount and saturation are separate observations: it illuminated whole blood in a capillary tube with red and infrared LEDs, detected scattered light and computed hemoglobin concentration and saturation from distinct signal relationships.<An oximeter for measuring hemoglobin concentration and oxygen content by A. P. Shepherd and others>[Abstract, apparatus and computed quantities]{ref}
For physiological transport, <coronary oxygen delivery> is flow times content. A pressure or saturation observation alone cannot supply that product without the remaining quantity information. The arterial sampling site, time and measurement definitions are part of the content result.
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