User:AJim/rbc

Recall that respiration, as illustrated schematically here with a unit of carbohydrate, produces about as many molecules of carbon dioxide, CO2, and it consumes of oxygen, O2. ^[1]

${\displaystyle {\ce {HCOH + O2 -> CO2 + H2O}}}$

Thus, the function of the circulatory system is as much about the transport of carbon dioxide as about the transport of oxygen. As stated elsewhere in this article, most of the carbon dioxide in the blood is in the form of bicarbonate ion. The bicarbonate provides a critical pH buffer^[2]. Thus, unlike hemoglobin for O2 transport, there is a physiological advantage to not having a specific CO2 transporter molecule.

Red Blood cells, nevertheless, play a key role in the CO2 transport process, for two reasons. First, because, besides hemoglobin, they contain a large number of copies of the enzyme carbonic anhydrase on the inside of their cell membrane. ^[3] Carbonic anhydrase, as its name suggests, acts as a catalyst of the exchange between carbonic acid and carbon dioxide (which is the anhydride of carbonic acid). Because it is a catalyst, it can affect many CO2 molecules, so it performs its essential role without needing as many copies as are needed for O2 transport by hemoglobin. In the presence of this catalyst carbon dioxide and carbonic acid reach an equilibrium very rapidly, while the red cells are still moving through the capillary. Thus it is the RBC that ensures that most of the CO2 is transported as bicarbonate. ^{[4] [5]} At physiological pH the equilibrium strongly favors carbonic acid, which is mostly dissociated into bicarbonate ion.^[6]

${\displaystyle {\ce {CO2 + H2O <=>> H2CO3 <=>> HCO3- + H+}}}$

The H+ ions released by this rapid reaction within RBC, while still in the capillary, act to reduce the oxygen binding affinity of hemoglobin, the Bohr effect.

The second major contribution of RBC to carbon dioxide transport is that carbon dioxide directly reacts with globin protein components of hemoglobin to form carbaminohemoglobin compounds. As oxygen is released in the tissues, more CO2 binds to hemoglobin, and as oxygen binds in the lung, it displaces the hemoglobin bound CO2, this is called the Haldane effect. Despite the fact that only a small amount of the CO2 in blood is bound to hemoglobin in venous blood, a greater proportion of the change in CO2 content between venous and arterial blood comes from the change in this bound CO2.^[7] That is, there is always an abundance of bicarbonate in blood, both venous and arterial, because of its aforementioned role as a pH buffer.

In summary, carbon dioxide produced by cellular respiration diffuses very rapidly to areas of lower concentration, specifically into nearby capillaries. ^{[8] [9]} When it diffuses into a RBC, CO2 is rapidly converted by the carbonic anhydrase found on the inside of the RBC membrane into bicarbonate ion. The bicarbonate ions in turn leave the RBC in exchange for chloride ions from the plasma, facilitated by the band 3 anion transport protein colocated in the RBC membrane. The bicarbonate ion does not diffuse back out of the capillary, but is carried to the lung. In the lung the lower partial pressure of carbon dioxide in the alveoli causes carbon dioxide to diffuse rapidly from the capillary into the alveoli. The carbonic anhydrase in the red cells keeps the bicarbonate ion in equilibrium with carbon dioxide. So as carbon dioxide leaves the capillary, and CO2 is displaced by O2 on hemoglobin, sufficient bicarbonate ion converts rapidly to carbon dioxide to maintain the equilibrium. ^{[10] [11] [12] [13]}