Compare and contrast the carriage of oxygen and carbon dioxide in blood.

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College Answer

Candidates who scored well for this question not only had a good knowledge of the topic 
but also displayed an organised approach to their answer through the use of a tabular 
format or some other structured approach. For a good answer, candidates were expected to 
provide information on the amount (both arterial and venous blood content, partial 
pressure) and form of carriage (binding to, loading and unloading from haemoglobin, 
dissolved, as bicarbonate, etc.) of oxygen and carbon dioxide in blood

Discussion

Carriage of oxygen in the blood is defined by the equation which describes total blood oxygen content

Total oxygen content of the blood = (sO2 × ceHb × BO) + (PaO2 × 0.003)

Where:

  • ceHb = the effective haemoglobin concentration
    • i.e. concentration of haemoglobin species capable of carrying and releasing oxygen appropriately
  • PaO2 = the partial pressure of oxygen in arterial gas
  • 0.003 = the content, in ml/L/mmHg, of dissolved oxygen in blood
    • Henry's law states that the amount of dissolved gas in a liquid is proportional to its partial pressure above the liquid;
    • Ergo the amount of oxygen dissolved in is proportional to its partial pressure, e.g for a PaO2 of 100 mmHg the oxygen content is 0.003 × 100 = 3ml/L
  • BO the maximum amount of Hb-bound O2 per unit volume of blood
    • normally 1.39 of dry Hb, or closer to 1.30 in "real" conditions
  • sO2 = oxygen saturation:
    • determined by the sigmoid oxygen-haemoglobin dissociation curve
    • Sigmoid shape of the curve comes from the positive cooperativityof oxygen binding
    • Once an oxygen molecule is bound to it, the oxygenated subunit increases the oxygen affinity of the three remaining subunits
    • This is because of a conformational change produced by each subunit binding oxygen, which mediates the transition from the T (tense, deoxygenated) state to the R (relaxed, oxygenated) state

Carriage of CO2 in the blood is 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

However, one might note that the college asked for a tabulated answer. That might be difficult to fill with enough detail to pass, but anyway here goes:

Comparison of Oxygen and Carbon Dioxide Transport
Transport mechanism Oxygen Carbon dioxide
Dissolved in blood

Minimal solubility (0.003/mmHg/L)

Less than 1% of total Hb carriage

20 times more soluble than O2

Complex with proteins

Complexed with haemoglobin

Each 1g of haemoglobin can carry about 1.3ml of oxygen

Oxygen saturation of haemoglobin is determined by the sigmoid oxygen-haemoglobin dissociation curve

Almost 100% of oxygen is transported in this fashion

Carbon dioxide can be transported as carbamino compounds

These are 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

Converted into another compound

Combined with water, COforms carbonic acid, which in turn forms bicarbonate:
CO2 + H2O ⇌ H2CO⇌ HCO3- + H+

This mainly happens in RBCS

References

References

Chapler, C. K., and S. M. Cain. "The physiologic reserve in oxygen carrying capacity: studies in experimental hemodilution." Canadian journal of physiology and pharmacology 64.1 (1986): 7-12.

Lifson, Nathan, et al. "The fate of utilized molecular oxygen and the source of the oxygen of respiratory carbon dioxide, studied with the aid of heavy oxygen."Journal of Biological Chemistry 180.2 (1949): 803-811.

Pittman, Roland N. Chapter 4 - Oxygen Transport; in "The Circulatory System and Oxygen Transport." (2011).

Willem G. Zijlstra. "Misconceptions in reporting oxygen saturation." Anesthesia & Analgesia 105.6 (2007): S5-S9.

Gregory, I. C. "The oxygen and carbon monoxide capacities of foetal and adult blood." The Journal of physiology 236.3 (1974): 625-634.

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.