Carbon dioxide transport in blood

CO2 is made as a waste product by cells, and blood helps to transport that CO2 from the tissues to the lungs - where we can breathe it out.

Now, to facilitate this - blood has a three important mechanisms to move CO2 around.

First, a small amount of CO2 is dissolved in the plasma - which is the liquid portion of blood.

Now, to calculate the concentration of dissolved carbon dioxide, you can multiply the partial pressure of CO2 (PCO2), measured in millimeters of mercury (mmHg), with the solubility of CO2.

The solubility of CO2 is the amount of CO2 that can be dissolved in blood, and it turns out that in a 100ml of blood, 0.07 mL of CO2 is dissolved per mmHg of CO2.

In venous blood, the equation becomes dissolved CO2 equals the venous partial pressure of CO2 (PVCO2) in mmHg times 0.07mL CO2, per mmHg, per 100mL blood.

And, if we plug in the partial pressure of CO2 in the veins, which is about 45 mmHg, we get 3.15 mL of CO2 in 100ml of blood.

This works out to be about 5% of the total CO2 transported by the blood.

Now another 3%, or about 1.89mL of CO2 in 100mL of blood, is transported a second way: CO2 binds directly to the terminal amino acids of each of the four globin chains in a hemoglobin protein.

Hemoglobin is the most abundant protein in the red blood cells, and each hemoglobin, can hold on to 4 molecules of CO2.

When hemoglobin is bound to CO2 it’s called carbaminohemoglobin.

Now, as carbaminohemoglobin alters the shape of the hemoglobin molecule slight and it decreases hemoglobin’s affinity for oxygen, and this is called the Bohr Effect.

It leads to slightly more O2 becoming unbound and getting dropped off in tissues full of CO2.

This causes a shift to the right in the oxygen-hemoglobin dissociation curve.

But the majority of CO2, about 90%, or about 56.7mL of CO2 in 100mL of blood, is transported a third way which involves turning CO2 into a bicarbonate ion (HCO3-).

To get there, CO2 first undergoes a chemical reaction with water to form carbonic acid (H2CO3).

As a weak acid, carbonic acid H2CO3 easily dissociates into hydrogen H+ ions and bicarbonate ions HCO3-.

And these reactions are reversible, and can happen in the opposite direction as well.

And while this reaction can also happen in the plasma, it is sped up in the red blood cell by the enzyme carbonic anhydrase to produce a large amount of HCO3- and H+.