Describe the carriage of carbon dioxide (CO2) in the blood.

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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, COforms carbonic acid, which in turn forms bicarbonate:
      CO2 + H2O ⇌ H2CO⇌ 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-NHand 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):

Difference between Arterial and Venous
Carbon Dioxide Content
  Arterial
(mmol/L)
Venous
(mmol/L)

Difference in mmol/L
(and % contribution to the a-v difference)

Dissolved 1.08 1.24 0.16   (9.5%)
Bicarbonate 18.53 19.97 1.44   (85.2%)
Carbamates 1.05 1.14 0.09   (5.3%)
Total (mmol/L) 20.66 22.35 1.69  (100%)

References

References

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.