HBO2

HbO2, or oxyhemoglobin, is the oxygenated form of hemoglobin, the iron-containing protein in red blood cells that transports oxygen. It forms when molecular oxygen binds to ferrous iron in the heme groups of hemoglobin. Each hemoglobin molecule has four heme sites and can carry up to four O2 molecules; binding is cooperative, producing a sigmoidal oxygen-hemoglobin dissociation curve. In the lungs, high PO2 favors loading of O2, converting deoxyhemoglobin to HbO2. In tissues, lower PO2 and the presence of CO2, H+, temperature changes, and 2,3-bisphosphoglycerate shift the equilibrium toward O2 release, delivering oxygen where needed. Structural changes between the tense (T) and relaxed (R) states underlie this cooperativity and affinity modulation.

Physiologically, the proportion of HbO2 in arterial blood is described by arterial oxygen saturation (SaO2) or, noninvasively, by SpO2 measured with pulse oximetry. The oxygen affinity is often summarized by P50, the partial pressure of oxygen at 50% saturation; higher P50 indicates lower affinity and easier unloading of oxygen to tissues. Neonatal and adult hemoglobins differ in affinity due to isoforms such as HbF and HbA, influencing HbO2 binding characteristics.

Clinical relevance includes conditions that alter HbO2 binding or delivery, such as carbon monoxide poisoning, which forms carboxyhemoglobin and reduces available HbO2, or methemoglobinemia, which impairs O2 transport despite adequate PaO2. HbO2 measurements, alongside dissolved oxygen, help assess oxygen delivery in health and disease.