Describe the carriage of carbon dioxide (CO2) in the blood.
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.
CO2 is transported by three (maybe, four) major mechanisms:
There is a difference between arterial and venous CO2 content:
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):
Arterial (mmol/L) |
Venous (mmol/L) |
Difference in mmol/L |
|
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%) |
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.