Oxygen binding capacity and oxygen content

Oxygen content is the amount of oxygen in a certain volume of blood, typically 100mL.

Oxygen binding capacity is the maximum amount of oxygen that can be bound to hemoglobin, abbreviated as Hb, which is the main protein found inside of red blood cells, which is a main component of blood.

As it turns out, there are two major ways for oxygen to move around in the blood.

The majority of oxygen is bound to hemoglobin inside red blood cells, and a small amount is dissolved directly in the blood plasma.

So the oxygen content of blood is the sum of these two, oxygen content equals hemoglobin-bound oxygen plus dissolved oxygen.

Now, if you just wanted to calculate dissolved oxygen, you’d do that by multiplying the partial pressure of oxygen, measured in mmHg, with the solubility of oxygen.

And the solubility of oxygen is the amount of oxygen that can be dissolved in 100mL of blood, and it has a constant value of 0.003 mL of O2, per mmHg per 100mL of blood.

So the equation becomes dissolved oxygen equals partial pressure of oxygen in mm of mercury times 0.003.

So if we plug in a physiologic arterial pressure of O2 of 100 mmHg, we get 0.3 ml of oxygen in 100ml of blood.

Now, that’s not enough oxygen to meet the metabolic demands of the body. So that’s where hemoglobin comes to the rescue.

The oxygen binding capacity of hemoglobin, is the maximum amount of oxygen in milliliters that 1 gram of hemoglobin can bind, multiplied by the number of grams of hemoglobin in 100mL of blood.

Each hemoglobin molecule can carry up to four molecules of O2, that’s one oxygen molecule for each of the four hemoglobin subunits.

Now, because each red blood cell carries a few hundred million hemoglobin proteins, that means that each red blood cell carries over a billion O2 molecules.

So that for each gram of hemoglobin there’s 1.34 mL O2 carried around.