Arterial Oxygen Content (CaO₂)

Blood carries oxygen two ways, and the split is lopsided. Each gram of haemoglobin binds up to 1.34 mL of oxygen when fully saturated, a figure known as Hüfner's number, and a much smaller amount dissolves directly in plasma at 0.003 mL per decilitre per mmHg. A healthy adult with 15 g/dL of haemoglobin, 98 percent saturation and a PaO₂ of 100 mmHg carries $1.34 \times 15 \times 0.98 = 19.7$ mL/dL bound and 0.3 mL/dL dissolved, near enough 20 mL/dL in total.

Those numbers make the important point on their own. The dissolved fraction is 1.5 percent of the total, which is why a pulse oximeter reading of 98 percent tells you almost nothing about how much oxygen a patient actually has. Halve the haemoglobin to 8 g/dL and the content falls to about 10 mL/dL even at a respectable saturation, and no amount of supplemental oxygen fixes it, because you are topping up the fraction that barely counts. Anaemia is an oxygen delivery problem that a saturation monitor cannot see.

The constants have small caveats. Hüfner's number is quoted variously as 1.34, 1.36 or 1.39 mL/g depending on whether you use the theoretical value or a measured one, and any dyshaemoglobin, carboxyhaemoglobin from smoke or methaemoglobin from certain drugs, is haemoglobin that cannot carry oxygen while still being counted in the total. A pulse oximeter is also blind to carboxyhaemoglobin and will read reassuringly high in carbon monoxide poisoning, where the content is catastrophically low. Content is also not delivery: multiply by cardiac output to get oxygen delivery, which is the quantity that keeps tissue alive.