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.


Sep 27, 2020 | Posted by in ANESTHESIA | Comments Off on Chapter 9 – Carbon Dioxide Transport
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