3. Physiology
        3.16. SAQs
            3.16.12. Respiratory
                3.16.12.4. Gas transport in the blood
3.16.12.4.2. CO2 carriage in blood

CO2 carriage in blood

Describe the ways in which CO2 is carried in the blood (99B5)
Describe the role of haemoglobin in the carriage of carbon dioxide in the blood (97A3)

Carbon dioxide is carried in 3 forms

  1. dissolved CO2
  2. bicarbonate
  3. carbamino compounds

1. Dissolved CO2

2. Bicarbonate

CO2 + H2O <-CA-> H2CO3 <-> H + HCO3

Chloride shift

In RBC, as H2CO3 is formed (with aid of CA), it breaks down to H + HCO3

=> HCO3 diffuse out easily

=> H+ doesn't really diffuse out because RBC membrane is relatively impermeable to cations

=> As [H+] builds up, chloride diffuse into RBC to maintain electrical neurality (in accordance to Gibbs-Donnan equilibrium)
aka "chloride shift"

[See diagram 6.5 in West p81]

Osmolarity increase, and water enters RBC as a result

=> slight increase in RBC volume as CO2 is taken up.

3. Carbamino compound

Imidazole group of histidine

Buffering

Both carbonic acid and carbamino haemogoblin almost completely dissociate

=> H+ are produced

=> buffered by histidine

As haemoglobin becomes reduced (i.e. deoxyHb)

=> it becomes less acid / better base

=> it becomes a better buffer

Haldane effect

Haldane effect refers to the increased ability of blood to carry CO2 when haemoglobin gives up oxygen.

Haldane effect is due to:

NB: Bohr effect refers to increased unloading of O2 from Hb when carbon dioxide is loaded
(causing pH drop, and thus right shift in oxygen dissociation curve)

CO2 dissociation curve

[See diagram...]

Carriage in arterial blood vs venous blood

Arterial blood contains 48 mLs of CO2 per 100mL blood.

Venous blood contains 52 mLs of CO2 per 100mL blood.

Total carriage

% Contribution to A-V difference

(i.e. % of CO2 eliminated at lung)

Additional notes

Due to storage of CO2 as bicarbonate, changes in PaCO2 due to changes in ventilation takes a little longer to equilibrate than PaO2.

 

Factors affecting PCO2 in steady state

Alveolar CO2 conc. x alveolar ventilation = CO2 output

Examiner's comment

 

To be added later

????Factors that move the CO2 dissociation curve

 

 

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Created20050225
Updated20050305


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