Describe how carbon dioxide (CO2) is carried in the blood.
It was expected answers would describe each of the main categories of how CO2 is carried:
Dissolved (10%), Plasma Bicarbonate (70%) and conjunction with plasma proteins and Hb as
Carbamino Hb (20%). An opening statement quantifying the amount of CO2 dissolved in
arterial (48mL/100mL) and venous blood (52mL/100mL) (4mL/100mL) and how this
compares with Oxygen was expected (20 times more soluble).
For dissolved CO2, the application and description of Henry’s Law was awarded marks.
A description of the consequences of the Haldane effect: difference in CO2 carriage of
oxygenated and deoxygenated blood was expected. A diagram of pCO2 v CO2 content was
That "opening statement" they ask for would probably sound something like this:
- The arterial blood CO2 content (480ml/L) is lower than the mixed venous CO2 content (520ml/L)
- This is substantially higher than the oxygen content of blood
- This is because CO2 is not only 20 times more water-soluble than oxygen, but also because it is carried in a number of different forms
CO2 is transported by three (maybe, four) major mechanisms:
- As bicarbonate (HCO3- ), 70-90% of total blood CO2 content
- Combined with water, CO2 forms carbonic acid, which in turn forms bicarbonate:
CO2 + H2O ⇌ H2CO3 ⇌ 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-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
- 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
There is a difference between arterial and venous CO2 content:
- Mixed venous blood has a total CO2 content of about 22.5 mmol/L
(or 520 mL/L)
- Arterial blood has a total CO2 content of about 20.5 mmol/L
- Much of this difference is due to the increase in bicarbonate concentration (85%)
- Some of this difference is also due to the Haldane effect:
- Deoxyhaemoglobin has about 3.5 times the affinity for CO2 when compared to oxyhaemoglobin
- This increases the CO2 binding capacity of venous blood
- Deoxyhaemoglobin is also a better buffer than oxyhaemoglobin, which increases the capacity of RBCs to carry HCO3-
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):
Difference between Arterial and Venous
Carbon Dioxide Content
Difference in mmol/L
(and % contribution to the a-v difference)
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