Oxygen Saturation
Oxygen Saturation:
Oxygen saturation (SO₂) is the percentage of functional hemoglobin saturated with oxygen. There is a popular curve (which can be seen below), that pops up a lot in clinical science.
On the x-axis is partial pressure of oxygen (PO₂), and on the y-axis is hemoglobin oxygen saturation percent (Hgb SO₂%). As partial oxygen pressure increases, so does hemoglobin oxygen saturation, and conversely when partial oxygen pressure decreases so does hemoglobin oxygen saturation.
Ok so what? The ‘B’ curve represents a normal oxygen saturation curve. The ‘A’ curve (shift to the left) represents the behavior of hemoglobin in the lungs. In the lungs there will be higher affinity for oxygen which makes sense because this is where hemoglobin loads up on oxygen but why does it work that way? Carbon dioxide and hydrogen ions effect the affinity of oxygen to hemoglobin. The low acid environment, ↑pH ↓CO₂, due to the breathing off of CO₂ and coolness due to the air temperature allow oxygen to load up onto hemoglobin at a lower pressure. See the below illustration for a graphic representation.
The opposite occurs in tissue. The ‘C’ curve (shift to the right) represents the behavior of hemoglobin in tissue. In tissue there is a high acid environment, ↓pH ↑CO₂ due to muscle tissue using up oxygen and giving off CO₂, and warmth due to tissue movement (think how your car warms up when you drive it). The high acid, high temperature environment makes hemoglobin want to drop off or dissociate from oxygen. This makes sense because the tissues need oxygen.
There’s one other important molecule to understand when talking about oxygen affinity for hemoglobin, 2,3-diphosphoglycerate (2,3-DPG). 2,3-DPG makes it easier for hemoglobin to drop off oxygen to tissue. 2,3-DPG will decrease the affinity of oxygen to hemoglobin in tissue. When the hemoglobin molecule goes back to the lung to pick-up more oxygen, the 2,3-DPG is dissociated.
Nerdy Note
If you go hiking at high altitudes your body will go through a shift to the right and increase 2,3-DPG. This process however is not instantaneous, it takes 2-3 days, and that’s why it’s recommended to acclimate as you increase in altitude. As you begin to acclimate, a number of physiological processes occur as well as the increase in 2,3-DPG which allows you to use oxygen more efficiently.