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
pressure) and form of carriage (binding to, loading and unloading from haemoglobin,
dissolved, as bicarbonate, etc.) of oxygen and carbon dioxide in blood
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 × BO2 ) + (PaO2 × 0.003)
Carriage of CO2 in the blood is by three (maybe, four) major mechanisms:
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:
This mainly happens in RBCS
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