What is the primary factor in oxygen release or attachment from hemoglobin?

Partial Pressure of Oxygen

The primary factor in oxygen release or attachment from hemoglobin is the partial pressure of oxygen (pO2). Hemoglobin has a sigmoid-shaped oxygen-binding curve, which means that the amount of oxygen bound to hemoglobin increases rapidly at low pO2 levels but then plateaus at higher pO2 levels. This shape is due to the cooperative binding of oxygen to hemoglobin, which means that the binding of one oxygen molecule to hemoglobin increases the affinity of the remaining subunits for oxygen.

At the lungs, where the pO2 is high, oxygen binds to hemoglobin and is transported to the tissues. In the tissues, where the pO2 is lower, oxygen is released from hemoglobin and diffuses into the cells. The Bohr effect, which is the decrease in hemoglobin affinity for oxygen at lower pH levels, also plays a role in oxygen release in the tissues.