Carbon dioxide transport in blood

Carbon dioxide is made as a waste product by cells, and blood helps to transport that carbon dioxide from the tissues to the lungs - where we can breathe it out. Now, to facilitate this - blood has three important mechanisms to move carbon dioxide around.

First, a small amount of carbon dioxide 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 carbon dioxide, measured in millimeters of mercury, with the solubility of carbon dioxide.

The solubility of carbon dioxide is the amount of carbon dioxide that can be dissolved in blood, and it turns out that in a 100 mL of blood, 0.07 mL of carbon dioxide is dissolved per millimeter of mercury of carbon dioxide.

In venous blood, the equation becomes dissolved carbon dioxide equals the venous partial pressure of carbon dioxide in millimeters of mercury times 0.07mL carbon dioxide, per millimeters of mercury, per 100 mL blood.

And, if we plug in the partial pressure of carbon dioxide in the veins, which is about 45 millimeters of mercury, we get 3.15 mL of carbon dioxide in 100 mL of blood.

This works out to be about 5% of the total carbon dioxide transported by the blood, but it can go up to 10%. Now another 10-20% is transported a second way: carbon dioxide 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 carbon dioxide. When hemoglobin is bound to carbon dioxide 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 oxygen becoming unbound and getting dropped off in tissues full of carbon dioxide. This causes a shift to the right in the oxygen-hemoglobin dissociation curve.

But the majority of carbon dioxide, about 70-80%, is transported a third way which involves turning carbon dioxide into a bicarbonate ion. To get there, carbon dioxide first undergoes a chemical reaction with water to form carbonic acid. As a weak acid, carbonic acid easily dissociates into hydrogen ions and bicarbonate ions. 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 bicarbonate ions and hydrogen ions.