This chapter is most relevant to Section F8(ii) from the 2017 CICM Primary Syllabus, which expects the exam candidates to be able to "describe the carbon dioxide carriage in blood including the Haldane effect and the chloride shift". The specific question of CO2 storage has never been as popular as the question of transport or carriage, and so this topic is under-represented in the CICM First Part exams. 

In summary:

Human bodily CO2 stores consist of:

Bone stores:

  • Part of the matrix of bone
  • As bicarbonate (30%)
  • As carbonate (70%)
  • Vast stores (1.6L/kg of body weight)

Of the bone stores:

  • about 9% are accessible to assist with buffering
  • ~ 200ml/kg, or 14L

Dissolved carbon dioxide

  • 3L are available for immediate use in buffering
  • 80-90% is stored in the form of bicarbonate anions (HCO3-)
  • 5-10% is present as unchanged gas, dissolved in the water of extracellular fluid (predominantly in the blood).
  • 5-10% or so is stored as carbamino compounds inside erythrocytes
  • 2-5% is available as a free gas in the alveolar gas mixture.
  • Miniscule amount available as carbonic acid

Whole-body CO2 stores

Much of one's CO2, tied up in non-dissolved form, can be viewed as a form of storage. Strange to think of it in that way (why would one possess a capacious reserve of a waste product?) but in fact the "stored" CO2 plays a major role in maintaining the acid-base balance.

How much CO2 do you have stored right now? To explore this topic in detail, an excellent article by Cherniack and Longobardo can be found in Physiology Reviews (1970) - but it is paywalled. Fortunately, the free preview at offers a brief glimpse. The information afforded by this glimpse has been taken by the author and again represented as one of those disgraceful gamblegrams:

Whole body CO2 stores

Thus, the adult 70kg male has about 1.8L/kg - up to 126L in total - of "potential" COtrapped inside them. A massive amount, about 1.6L/kg, is not readily accessible, as it is a part of the crystalline matrix of bone, or dissolved in the cytosol of tissues. The bony stores consist of about 30% bicarbonate, and 70% carbonate; they were revealed to researchers who heated bone and observed the release of CO2 (which was in considerable volume). This form of storage is largely irrelevant to the minute and second timescales of respiratory acid-base physiology, but becomes more important in chronic disorders, and plays a considerable role in the certain exotic circumstances-for example, in the demineralisation of bone associated with decompression sickness.

Of this massive bony storehouse, about 9L is gradually exchanged, and forms 15% of the total daily CO2 production; this exchange process probably plays little role in buffering because of its glacial slowness (its rate appears to be limited by blood flow to bone). The remainder is only about 3L - this is the CO2 in circulation - is involved in routine gas exchange processes and buffering; this is the CO2we are interested in when we discuss the compensation for acute acid-base disturbances.

So, it is possible to arrive at a volume of carbon dioxide which is available reasonably quickly. A confident-sounding radiolabelled bicarbonate study presents us with a figure of around 200 (+/- 42) ml/kg of CO2 for adults. The average adult male therefore has about 14 litres of CO2 on board which is fairly easy to access on the timescales involved in subacute acid-base disorders, and of these 14L about 3L are available for immediate use in buffering.

Of these 3 litres, the majority (80-90%) is stored in the form of bicarbonate anions (HCO3-) swimming around in extracellular fluid. About 5-10% is present as unchanged gas, dissolved in the water of extracellular fluid (predominantly in the blood). Another 5-10% or so is stored as carbamino compounds inside erythrocytes, and a minor fraction (perhaps 2-5%) is available as a free gas in the alveolar gas mixture. Lastly, the smallest fraction of all - measured probably in millilitres or microlitres- exists at any given time in the form of carbonic acid, and represents a transitional form; this is either CO2 in the process of being turned into bicarbonate, or bicarbonate in the process of being turned into CO2.


Martinu, Tereza, Dick Menzies, and Sandra Dial. "Re-evaluation of acid-base prediction rules in patients with chronic respiratory acidosis." Canadian respiratory journal: journal of the Canadian Thoracic Society 10.6 (2003): 311-315.

Jones, Norman L. "Respiratory acidosis sans acidemia." Canadian respiratory journal: journal of the Canadian Thoracic Society 10.6 (2003): 301-303.

Berend, Kenrick, Aiko PJ de Vries, and Rijk OB Gans. "Physiological approach to assessment of acid–base disturbances." New England Journal of Medicine371.15 (2014): 1434-1445.

Dorman, Philip J., W. James Sullivan, and Robert F. Pitts. "The renal response to acute respiratory acidosis." Journal of Clinical Investigation 33.1 (1954): 82.

Schwartz, William B., Adrien Falbriard, and Guy Lemieux. "The kinetics of bicarbonate reabsorption during acute respiratory acidosis." Journal of Clinical Investigation 38.6 (1959): 939.

Giebisch, Gerhard, et al. "The extrarenal response to acute acid-base disturbances of respiratory origin." Journal of Clinical Investigation 34.2 (1955): 231.

Brackett Jr, Newton C., Jordan J. Cohen, and William B. Schwartz. "Carbon dioxide titration curve of normal man: Effect of increasing degrees of acute hypercapnia on acid-base equilibrium." New England Journal of Medicine 272.1 (1965): 6-12.

Schwartz, William B., Newton C. Brackett Jr, and Jordan J. Cohen. "The response of extracellular hydrogen ion concentration to graded degrees of chronic hypercapnia: the physiologic limits of the defense of pH." Journal of Clinical Investigation 44.2 (1965): 291.

Goldstein, Marc B., F. John Gennari, and William B. Schwartz. "The influence of graded degrees of chronic hypercapnia on the acute carbon dioxide titration curve." Journal of Clinical Investigation 50.1 (1971): 208.

Alexander, James K., et al. "Analysis of the respiratory response to carbon dioxide inhalation in varying clinical states of hypercapnia, anoxia, and acid-base derangement." Journal of Clinical Investigation 34.4 (1955): 511.

Harned, Herbert S., and Raymond Davis Jr. "The ionization constant of carbonic acid in water and the solubility of carbon dioxide in water and aqueous salt solutions from 0 to 50." Journal of the American Chemical Society 65.10 (1943): 2030-2037.

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

Forster, R. E., and N. Itada. "Carbonic anhydrase activity in intact red cells as measured by means of 18O exchange between CO2 and water." Biophysics and Physiology of Carbon Dioxide. Springer Berlin Heidelberg, 1980. 177-183.

Loerting, Thomas, and Juergen Bernard. "Aqueous carbonic acid (H2CO3)."ChemPhysChem 11.11 (2010): 2305-2309.

Hage, Wolfgang, Andreas Hallbrucker, and Erwin Mayer. "Carbonic acid: synthesis by protonation of bicarbonate and FTIR spectroscopic characterization via a new cryogenic technique." Journal of the American Chemical Society 115.18 (1993): 8427-8431.

Moore, M. H., R. L. Hudson, and R. F. Ferrante. "Radiation products in processed ices relevant to Edgeworth-Kuiper-Belt objects." The First Decadal Review of the Edgeworth-Kuiper Belt. Springer Netherlands, 2004. 291-306.

Baer, E., and D. M. Reith. "Acetazolamide poisoning in a toddler." Journal of paediatrics and child health 37.4 (2001): 411-412.

Altay, S. "Acetazolamide overdose." Reactions 1510 (2014): 5-19.

Armon, Yaacov, et al. "Oral [13C] bicarbonate measurement of CO2 stores and dynamics in children and adults." Journal of Applied Physiology 69.5 (1990): 1754-1760.

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.

Poyart, C. F., A. Freminet, and E. Bursaux. "The exchange of bone CO2 in vivo." Respiration physiology 25.1 (1975): 101-107.

Poyart, C. F., E. Bursaux, and A. Freminet. "The bone CO2 compartment: Evidence for a bicarbonate pool." Respiration physiology 25.1 (1975): 89-99.

Schaefer, Karl E. "Involvement Of Co2 And Calcium Stores In Decompression Sickness." (1973). Naval Submarine Medical Tesearch Laboratory Report Number 738.Bureau of Medicine and Surgery, Navy Department Research Work Unit MR041. 01.01-0063BOKL.08