Confused About Carbon Dioxide Transport?

The blood not only transports oxygen from the lungs to the tissues but also has the very important function of transporting carbon dioxide in the reverse direction, from the tissues to the lungs. Carbon dioxide is transported in three ways: (1) it is carried as gas dissolved in the plasma; (2) some is carried in loose combination with hemoglobin in the red blood cells; and (3) most of it is carried as bicarbonate ions in the red blood cells and plasma.

The affinity of hemoglobin for oxygen is strongly influenced by pH, because H+ ions act as a negative allosteric modulators for hemoglobin. As the pH decreases, the affinity of hemoglobin for oxygen decreases, a response called the Bohr effect. The pH of the blood in the tissues is low because CO₂ released from the cells combines readily with water to form carbonic acid (H₂CO₃), most of which ionizes into hydrogen (H⁺) and bicarbonate (HCO_3-) ions. Red blood cells contain an enzyme (carbonic anhydrase) that greatly speeds up this reaction. The result is increased acidity in the tissues:

CO₂ + H₂O ---------> H₂CO₃ ----------> H⁺ + HCO_3-

Thus oxyhemoglobin in the tissue capillaries is exposed to an acid environment, where its affinity for O₂ is reduced, and it readily unload its O₂. The acidity of the blood must be buffered however, and here again hemoglobin plays a role. Deoxyhemoglobin readily binds H⁺ ions:

H⁺ + HCO_3- + Hb:4O₂---------> HHb + HCO_3-

Most of the carbon dioxide is thus transported as bicarbonate ions, and the hydrogen ions are combined with hemoglobin and other plasma proteins. This is an example of the buffering action of proteins, without which the pH of the blood would drop to life-threatening levels. In the lungs, where carbon dioxide pressure is low and acidity if high, the reactions shown above are reversed, and carbon dioxide moves from the blood into the alveoli.

Summary of gas exchange chemistry in tissue capillaries. In tissues, where the partial pressure of CO₂ is high (and hence the pH is low) and the partial pressure of O₂ is low, CO₂ enters the plasma (yellow). Most of this CO₂ then diffuses into corpuscles (pink), where it is primarily converted into bicarbonate, most of which diffuses back into the plasma (not shown). The hydrogen ions thus liberated help alter the quaternary structure of hemoglobin from that of ordinary hemoglobin (Hb) to that of acid hemoglobin (hemoglobin bound to a hydrogen ion, or Hhb) As a result, the molecule loses much of its affinity for oxygen; O₂ is therefore released and diffuses into the plasma and from there into the tissues. In the pulmonary capillaries of the lung, where the partial pressures and pH are reversed, all of these reactions run the other way. A small quantity of CO₂ binds directly to hemoglobin (large black arrow), and an even smaller amount of CO₂ dissolves directly in the blood (yellow).