CO2 transport Flashcards
Where is CO₂ produced?
◆ CO₂ is produced in the tissues as a by-product of aerobic metabolism.
◆ It is then transported from the tissues to the lungs, where it is eliminated.
Compare CO₂ production rate and O₂ consumption rate.
◆ 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, CO production can rise as high as 4000 mL/min.
How is CO₂ transported
in the circulation?
◆ Dissolved in plasma;
◆ Bound to Hb and other proteins as carbamino compounds;
◆ As bicarbonate.
Describe CO₂ transportation as it is dissolved in plasma.
The [.] of CO₂ in solution is given by Henry’s law:
◆ The [.] of CO₂ in blood is the partial pressure multiplied by the solubility of CO₂;
◆ The solubility of CO₂ is 0.07 mL CO₂/100 mL blood per mm Hg;
◆ Thus the [.] of dissolved CO₂ in arterial blood, as calculated by Henry’s law, is 2.8 mL CO₂/100 mL blood (40 mm Hg × 0.07 mL CO₂/100 mL blood per mm Hg), which is approximately 5% of the total CO₂ content of blood.
Recall that because of the lower solubility of O₂, compared with CO₂, dissolved O₂ is only 2% of the total O₂ content of blood.
Describe CO₂ transportation bound to Hb and other proteins as carbamino compounds.
◆ It accounts for about 3% of the total CO₂.
◆ CO₂ binds to terminal amino groups on proteins within the Hb molecule;
◆ The amine groups involved are the side chains of arginine and lysine within the globin chains.
In CO₂ transportation as carbamino compounds, describe the Bohr and Haldane effect.
Bohr effect
◆ CO₂ binding to Hb reduces its affinity for O₂ and causes a right shift of the O₂-Hb dissociation curve;
Haldane effect
◆ The above causes O₂ bound to Hb to change its affinity for CO₂, such that when less O₂ is bound, the affinity of Hb for CO₂ increases.
◆ The metabolic waste products are therefore efficiently transported away from the tissues to the lungs.
Describe CO₂ transportation as HCO₃⁻.
◆ Accounts for more than 90% of the total CO₂.
◆ CO₂ + H₂O ←→ H₂CO₃ ←→ H⁺ + HCO₃⁻
◆ In the tissues, CO₂ generated from aerobic metabolism is added to systemic capillary blood, converted to HCO₃⁻ by the reactions described previously, and transported to the lungs.
◆ In the red blood cells, H₂CO₃ dissociates into H⁺ and HCO₃⁻.
◆ The H⁺ remains in the red blood cells, where it will be buffered by deoxyhemoglobin, and the HCO₃⁻ is transported into the plasma in exchange for Cl⁻ (chloride).
◆ All of the reactions previously described occur in reverse in the lungs H⁺ is released from its buffering sites on deoxyHb HCO₃⁻ enters the red blood cells in exchange for Cl⁻, H⁺ and HCO₃⁻ combine to form H₂CO₃, and H₂CO₃ dissociates into CO₂ and H₂O.
◆The regenerated CO₂ and H₂O are expired by the lungs.
◆ In the lungs, HCO₃⁻ is reconverted to CO₂ and expired.
What is the clinical relevance of blood gas changes in apnoea?
➜ In total, the circulation and lungs contain approximately 2.5 L of immediately available CO₂ and 1550 mL of O₂.
◆ If a healthy patient stops breathing (e.g. on induction of general anaesthesia), basal processes will continue: 250 mL/min of O₂ will be consumed and 200 mL/min of CO₂ will be produced.
◆ Therefore PCO₂ will ⬆︎.
◆ PO₂ will ⬇︎.
◆ Typically, SaO₂ falls to 70% (PO₂ 5.0 kPa) after 2 min.
► If the patient breathes O₂ for sufficient time to completely de-nitrogenate their FRC prior to the period of apnoea, the quantity of stored O₂ ⬆︎ to over 3 L.
► Even after 5 min of apnoea, SaO₂ will remain at 100%.
◉ Basal metabolic processes will continue, and after 5 min the PaCO₂ will approach 10 kPa.
