Question 5

College Answer

A definition of arterial and venous CO₂ content (mls and partial pressure) and an outline of the 3 forms of CO₂ in the blood and their contribution to the AV difference, followed by a detailed explanation of each form of carriage was required for this question. A good answer included a table of the contribution of each form of carriage to arterial and venous content and the AV difference; explained the concepts of chloride shift when describing carriage as HCO₃ - ; detailed the Haldane effect and its contribution to carbamino carriage and referenced Henry’s law when describing dissolved CO₂.

West’s Chapter 6 on gas transport details the key information to score well on this question.

Discussion

CO₂ is transported by three (maybe, four) 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 contributor 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^-

Though this answer is already growing overlong, one cannot help but add a "a table of the contribution of each form of carriage to arterial and venous content and the AV difference". This is put together using data from Geers & Gross (2000):

Geers, Cornelia, and Gerolf Gros. "Carbon dioxide transport and carbonic anhydrase in blood and muscle." Physiological Reviews 80.2 (2000): 681-715.

Farhi, L. E., and H. Rahn. "Gas stores of the body and the unsteady state."Journal of applied physiology 7.5 (1955): 472-484.

Cherniack, NEIL S., and G. S. Longobardo. "Oxygen and carbon dioxide gas stores of the body." Physiol Rev 50.2 (1970): 196-243.

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.