Hemoglobin-concentration and saturation separation

Hemoglobin-concentration and saturation separation is the distinction between how much hemoglobin a volume of blood contains and what fraction of its available oxygen-binding capacity is occupied. Oxygen saturation is a ratio; hemoglobin concentration is an amount per volume. Neither quantity alone specifies arterial oxygen content.

Let $H$ be total hemoglobin concentration, $F$ the fractional oxyhemoglobin and $\beta$ a chosen oxygen capacity per unit hemoglobin mass. A simple content model iswhere the final term represents dissolved oxygen using a solubility coefficient $\alpha$ and oxygen partial pressure $p_{{\mathrm{O}}_2}$. The model separates carrier amount, carrier occupancy and dissolved gas. Functional oxygen saturation can replace $F$ only when the excluded hemoglobin species are negligible or separately accounted for.^[ref][ref]

For an original comparison, neglect dissolved oxygen and stipulate $\beta =1.34$ mL/g and $F=0.98$. Samples with $H=15$ and $H=8$ g/dL then carry about $19.70$ and $10.51$ mL of bound oxygen per dL, respectively. The equal oxygen saturation does not imply equal oxygen content. Conversely, halving $H$ and doubling $F$ preserves the product $HF$ only while the new fraction remains physically attainable.

A primary instrument illustrates how the distinction can be measured rather than merely defined. In 1989, a capillary-tube oximeter used red and infrared light, a detector and digitized processing to derive hemoglobin concentration and oxygen saturation separately.^[ref] Measurements sensitive to different optical features supply information that one saturation number cannot contain. At fixed blood flow, oxygen delivery scales with content; equal saturation readings alone cannot establish equal delivery.