Oxygen is transported in the blood in two ways:
A small amount of O 2 (1.5 percent) is carried in the plasma as a dissolved gas.
Most oxygen (98.5 percent) carried in the blood is bound to the protein hemoglobin in red blood cells. A fully saturated oxyhemoglobin (HbO 2) has four O 2 molecules attached. Without oxygen, the molecule is referred to as deoxyhemoglobin (Hb).
The ability of hemoglobin to bind to O 2 is influenced by the partial pressure of oxygen. The greater the partial pressure of oxygen in the blood, the more readily oxygen binds to Hb. The oxygen‐hemoglobin dissociation curve, shown in Figure 1, shows that as pO 2 increases toward 100 mm Hg, Hb saturation approaches 100 percent. The following four factors decrease the affinity, or strength of attraction, of Hb for O 2 and result in a shift of the O 2‐Hb dissociation curve to the right:
- Increase in partial pressure of CO 2 (pCO 2).
- Increase in acidity (decrease in pH). The decrease in affinity of Hb for O 2, called the Bohr effect, results when H + binds to Hb.
- Increase in BPG (bisphosphoglycerate) in red blood cells. BPG is generated in red blood cells when they produce energy from glucose.
Figure 1. The oxygen-hemoglobin dissociation curve.