The oxygen-hemoglobin dissociation curve shows how the hemoglobin saturation with oxygen (SO2,), is related to the partial pressure of oxygen in the blood (PO2).
Hemoglobin is the main protein within red blood cells, and it’s made of four globin subunits, each containing a heme group capable of binding one molecule of O2.
So each hemoglobin protein can bind 4 molecules of oxygen. But each hemoglobin isn’t always 100% saturated or bound by oxygen.
A hemoglobin molecule might have no oxygen bound, and be 0% saturated, called deoxyhemoglobin, and it will take on a tense state shape, or T-state; or it might have one oxygen bound and three open spots, meaning that particular protein would be 25% saturated; or two filled spots and two open spots—50%; or 3 spots filled and one spot open—75%, or all spots filled and 100% saturated.
All of these states - where oxygen is bound to hemoglobin - are called oxyhemoglobin, changing to its relaxed state, or R-state with each O2 molecule that binds.
And since there are millions of hemoglobin molecules in a single cell and millions of red blood cells, the hemoglobin saturation of oxygen is the average saturation among all of these proteins.
Now it turns out that hemoglobin absorbs different wavelengths of light as it gets more and more oxygenated.
A technique called pulse oximetry uses this property of hemoglobin to figure out the average oxygen saturation across millions of hemoglobin proteins.
The main factor that influences oxygen saturation is the partial pressure of oxygen in the blood, measured in millimeters of mercury (mm Hg).
So for example, at a partial pressure of 25mmHg, hemoglobin proteins might be 50% saturated, called P50; and at a partial pressure of 100mmHg, they might be 98% saturated, meaning most are fully saturated.
And when these points are plotted, the curve takes on a sigmoidal shape.
In practical terms, this sigmoidal shape means that hemoglobin has an increasing affinity for O2 as the number of bound O2 molecules goes up.
So binding that 4th O2 molecule is much easier than binding that first O2 molecule. This is called positive cooperativity.
Around 60mmHg, the vast majority of the hemoglobin subunits have bound oxygen, so the curve starts to level off.
That’s why in arterial blood where the partial pressure of oxygen is around 100mmHg, hemoglobin get fully saturated with oxygen.
And why in the venous capillaries of tissues, where the partial pressure of oxygen is about 40mmHg, hemoglobin is only about 75% saturated with oxygen.
In other words, about a quarter of the oxygen that’s bound to the hemoglobin gets dropped off, or unloaded, in the tissues.
Now, there are a few factors that can cause hemoglobin’s affinity for O2 to change.
For example, when CO2 is produced during aerobic metabolism in the tissues, it dissolves into the blood plasma, increasing the PCO2, and increasing the amount of CO2 that gets inside the red blood cells.
Inside the red blood cell, the enzyme carbonic anhydrase catalyzes a reaction with CO2 and water which forms carbonic acid (H2CO3).
Carbonic acid (H2CO3) then splits into a bicarbonate ion (HCO3-) and a hydrogen H+ ions (H+).
As more and more H+ start to pile up, the pH starts to fall.
Now, both CO2 and H+ can bind to hemoglobin.
CO2 binds to the terminal amino acids in the globin subunits, forming carbaminohaemoglobin; while H+ bind to amino acid side chains that make up the globin subunits.
And even though CO2 and H+ don’t compete for the same binding sites as O2 in hemoglobin, they do stabilize the T-state of hemoglobin.
This results in a decrease in hemoglobin’s affinity for oxygen, forcing the unloading of O2.
So, for example, when muscles are working hard, like during exercise, they are producing a lot more CO2, and, as a result, more H+ , which causes the pH to drop. So O2 is unloaded in the issues that need it most.
Another factor is 2,3-diphosphoglyceric acid (2,3-DPG), which is a metabolic byproduct of glycolysis in red blood cells.