Chapter 9 – Carbon Dioxide Transport

Abstract

CO2 is produced in the tissues as a by-product of aerobic metabolism. One of the important roles of the circulation is to transport CO2 from the tissues to the lungs, where it is eliminated.

Chapter 9 Carbon Dioxide Transport

How does carbon dioxide production and storage compare with that of oxygen?

CO₂ is produced in the tissues as a by-product of aerobic metabolism. One of the important roles of the circulation is to transport CO₂ from the tissues to the lungs, where it is eliminated.

A typical adult produces CO₂ at a basal rate of 200 mL/min (at standard temperature and pressure), a slightly lower rate than the basal O₂ consumption (250 mL/min). During vigorous exercise, CO₂ production can rise as high as 4000 mL/min.

As discussed in Chapter 8, the body contains only 1.5 L of O₂. In contrast, an estimated 120 L of CO₂ is stored throughout the body in various forms.

How is carbon dioxide transported in the circulation?

CO₂ is transported in the circulation in three forms:

Dissolved in plasma. Like O₂, the volume of CO₂ dissolved in the plasma is proportional to the partial pressure of CO₂ above it (according to Henry’s law). Dissolved CO₂ makes a much greater overall contribution to total CO₂ carriage than dissolved O₂ does to O₂ carriage, because the solubility coefficient of CO₂ is 20 times greater than that of O₂.
Bound to Hb and other proteins as carbamino compounds. Not to be confused with COHb (carbon monoxide bound to Hb), carbaminohaemoglobin is a compound formed when CO₂ reacts with a terminal amine group within the Hb molecule. The amine groups involved are the side chains of arginine and lysine within the globin chains: CO₂ + HbNH₂ → HbNHCOOH – this is carbaminohaemoglobin. Deoxyhaemoglobin forms carbamino compounds more readily than does oxyhaemoglobin (see Haldane effect below).
As bicarbonate. The enzyme carbonic anhydrase (CA) catalyses the reaction between CO₂ and water to form H₂CO₃. The cytoplasm of red blood cells (RBCs) contains ample CA, whereas CA is absent in plasma. The reaction between CO₂ and water can therefore only occur within the RBC. Almost all the H₂CO₃ then dissociates into HCO₃‾ and protons (H⁺):

CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃‐
CO2+H2O⇌H2CO3⇌H++HCO3‾
CO₂ and water are able to directly diffuse through the RBC membrane, whilst H⁺ and HCO₃‾ cannot. As the CA reaction between CO₂ and water is an equilibrium reaction, it would cease if the H⁺ or HCO₃‾ formed were allowed to build up within the RBC. This is prevented by two processes:
1. – Chloride shift (or Hamburger effect). HCO₃‾ is transported across the RBC membrane down its electrochemical gradient by a specific transmembrane Cl‾/ HCO₃‾ exchanger. Therefore, while the HCO₃‾ ions leave the RBC, joining the blood bicarbonate buffer system, chloride ions enter the RBC (Figure 9.1) to maintain electrical neutrality.
2. – Binding of H⁺ to histidine residues. As H⁺ cannot cross the cell membrane of the RBC, it instead binds to histidine side chains of the Hb molecule, thereby reducing the intracellular concentration of H⁺ and facilitating the Bohr shift. Deoxyhaemoglobin is able to bind H⁺ ions better than is oxyhaemoglobin (see the Haldane effect below).
By keeping the levels of HCO₃‾ and H⁺ in the RBC low, the reaction between CO₂ and water proceeds and there is a continual conversion of CO₂ to HCO₃‾. The net effect of both processes is that a molecule of CO₂ produced by the tissues results in the addition of a Cl‾ ion to the RBC, whereas the H⁺ is bound and HCO₃‾ is removed to the extracellular fluid. CO₂ is not osmotically active but Cl‾ is; following the chloride shift, there is a small entry of water into the RBC. This is why venous RBCs have a 3% higher volume than arterial RBCs.