Fact of the Day: CO2 transport to the lungs

Fact:

When the tissues give of CO2 after undergoing metabolic oxidation, the CO2 enters into RBCs. This CO2 + H2O (with the help of carbonic anhydrase**) is converted into HCO3- and H+. The HCO3- is exchanged for Cl- (chloride shift), and drifts out into the plasma. This explains why the major form of CO2 is HCO3-! (80-90%)

So now the RBC is stuck with H+ in its cytoplasm. It turns out hemoglobin is a good buffer, but only in the deoxyhemoglobin form. Luckily, most of the O2 attached to hemoglobin has been released into the peripheral tissues, leaving a majority in the deoxyhemoglobin form, which readily attaches to H+.

When the RBC makes it to the lungs, the opposite happens. HCO3- is exchanged back into the RBC for Cl-, combines with H+ (again, with the help of carbonic anhydrase**), and reforms CO2 + H2O. The CO2 happily diffuses out into the alveolus, and the O2 enters back into the RBC to oxygenate hemoglobin.

So note, Cl- plays a crucial role in CO2 transport to the lungs.

**Technically, carbonic anhydrase is used for H2O + CO2 <--> H2CO3, and H2CO3 readily dissociates into HCO3- and H+