Question 1

College Answer

A detailed understanding of the carriage of carbon dioxide (CO2) in the blood is essential to the
practice of intensive care medicine. Comprehensive answers classified and quantified the
mechanisms of CO2 carriage in the blood and highlighted the differences between the arterial and
venous systems. An explanation of the physiological principles surrounding these differences and
the factors which may affect them was expected. The changes that occur at the alveolar and
peripheral tissue interfaces with a similar explanation of process was also required. Candidate
answers were often at the depth of knowledge required for an ‘outline question’ and a more
detailed explanation was required to score well

Discussion

 "The changes that occur at the alveolar and peripheral tissue interfaces" here are probably referring to the Bohr and Haldane effects. 

CO₂ is transported by three major mechanisms:

• As bicarbonate (HCO₃^- ), 70-90% of total blood CO₂ content
  • Combined with water, CO₂ forms carbonic acid, which in turn forms bicarbonate:
    CO₂ + H₂O ⇌ H₂CO₃ ⇌ HCO₃^- + H⁺
  • This mainly happens in RBCS
  • The rise in intracellular  HCO₃^- leads to the exchange of bicarbonate and chloride, the chloride shift. Chloride is taken up by RBCSs, and bicarbonate is liberated.
  • Thus chloride concentration is lower in systemic venous blood than in systemic arterial blood
• As carbamates, the conjugate bases of carbamino acid (about 10-20%)
  • Dissociated conjugate bases of carbamino acids, which form in the spontaneous reaction of R-NH₂ and CO₂.
  • Intracellular (RBC) carbamino stores are the greatest: haemoglobin, particularly deoxygenated haemoglobin, has a high affinity for CO₂, whereas most other proteins do not
• As dissolved CO₂ gas, about 10%
  • Henry's law states that the amount of dissolved gas in a liquid is proportional to its partial pressure above the liquid
  • Thus, for every 1 mmHg of pCO₂ the blood concentration increases by about 0.03 mmol/L
  • Thus, CO₂ is 10-20 times more soluble than oxygen 
• Carbonic acid: 
  • A miniscule proportion of total carbon dioxide exists in this form, i.e. it is not a major contributr to CO₂ transport

There is a difference between arterial and venous CO₂ content:

• Mixed venous blood has a total CO₂ content of  about 22.5 mmol/L
  (or 520 mL/L)
• Arterial blood has a total CO₂ content of about 20.5 mmol/L 
  (or 480ml/L)
• Much of this difference is due to the increase in bicarbonate concentration (85%)
• Some of this difference is also due to the Haldane effect:
  • Deoxyhaemoglobin has about 3.5 times the affinity for CO₂ when compared to oxyhaemoglobin
  • This increases the CO₂ binding capacity of venous blood
  • Deoxyhaemoglobin is also a better buffer than oxyhaemoglobin, which increases the capacity of RBCs to carry HCO₃^-

Arthurs, G. J., and M. Sudhakar. "Carbon dioxide transport." Continuing Education in Anaesthesia Critical Care & Pain 5.6 (2005): 207-210.

Klocke, Robert A. "Carbon dioxide transport." Comprehensive Physiology (2011): 173-197.

Groeneveld, AB Johan. "Interpreting the venous-arterial PCO2 difference." Critical care medicine 26.6 (1998): 979-980.