The oxygen dissociation curve describes the:

• A. pH value at saturation
• B. Concentration of DPG
• C. Ratio of adult Hb to fetal Hb
• D. Percent of Hb saturated with O2 at a given PaO2

Answer: (D)

The oxygen dissociation curve shows how much oxygen the hemoglobin is carrying at different levels of partial pressure of oxygen in the blood. In other words, it plots the percent of hemoglobin saturated with O2 as PaO2 changes. This relationship explains how hemoglobin binds oxygen in the lungs when PaO2 is high and releases it in tissues where PaO2 is lower. The curve has a characteristic S-shape due to cooperative binding: once one oxygen molecule binds, the remaining sites become more willing to bind O2, making saturation rise rapidly as PaO2 increases in the lungs, then level off as it nears full saturation.

Understanding this helps interpret how changes in PaO2 translate into oxygen delivery to tissues. For example, at a typical arterial PaO2 around 100 mmHg, hemoglobin is nearly fully saturated; at a tissue PaO2 around 40 mmHg, saturation drops, allowing oxygen to unload where it’s needed. The curve can shift left or right based on factors like pH, temperature, 2,3-DPG, and fetal versus adult hemoglobin, which alters the affinity of hemoglobin for oxygen, but the fundamental description remains: it reflects the percent Hb saturation at a given PaO2.

So the curve specifically describes the percent of hemoglobin saturated with O2 at a given PaO2.