oxygen content

Saturation tells you what fraction of the seats on a bus are filled; oxygen content tells you how many passengers are actually on board. It is the total amount of oxygen carried in a given volume of blood — usually expressed as millilitres of oxygen per 100 mL of blood — and it is the quantity that ultimately matters for keeping tissues alive.

Oxygen rides in blood two ways. The overwhelming majority is bound to hemoglobin inside red cells; a much smaller amount is dissolved directly in the plasma. Content therefore depends on three things together: how much hemoglobin there is, how saturated that hemoglobin is, and the dissolved oxygen pressure. Because hemoglobin carries so much more, the hemoglobin level and its saturation dominate the total, while the dissolved part contributes only a sliver under normal conditions.

This three-part dependence is why a normal saturation can still hide dangerously low oxygen delivery. A severely anaemic person may have hemoglobin that is 100 percent saturated, yet because there is so little hemoglobin to fill, the actual oxygen content — and thus the oxygen reaching the tissues — is low. This is also why blood transfusion, by adding hemoglobin, can raise oxygen content even when saturation was already full.