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

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
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:

 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-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 Converted into another compound - Combined with water, CO2 forms carbonic acid, which in turn forms bicarbonate: CO2 + H2O ⇌ H2CO3 ⇌ HCO3- + H+ This mainly happens in RBCS

### References

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