2019A04 Describe the ways in which carbon dioxide is carried in the blood.

 

List:

·         Key numbers

·         Carriage

·         Variable CO2 affinity

 

Key numbers:

 

Arterial

A-V difference

As HCO3-

90%

60%

As carbamino compound

5%

30%

As dissolved CO2

5%

10%

 

 

 

Tension

40mmHg

6mmHg

Content

48mL/100mL

4mL/100mL

 

Carriage:

As HCO3-

·   Location:

o Haemoglobin (140g/L, 39 imidazoles per molecule)

o Plasma proteins (70g/L, 13 imidazoles per molecule)

·   Purpose:

o Maintain partial pressure gradient for CO2 from tissues into capillary blood

·   Mechanism:

o CO2 + H2O < -> H+ + HCO3- (catalysed by carbonic anhydrase in RBC)

o KHb + H+ <-> HHb + K+ (buffering by imidazole groups of histidine residues)

o HCO3- exchanged for Cl- at membrane (Hamburger effect, facilitates above reactions)

As carbamino compound

·   Location: same

·   Purpose: same

·   Mechanism: NH2 + CO2 <-> NHCOO- + H+

As dissolved

·   Amount dissolved = PaCO2 x k (Henry’s law)

·   Solubility coefficient k: 0.03mmol/L/mmHg at 37°C

·   Note ↑temp -> ↓solubility -> ↑PaCO2:CaCO2

Illustration

 

Variable CO2 affinity:

Haldane effect

·   Higher uptake of CO2 and H+ in HHb c.f. HbO2:

o 70%: due to 3.5x ↑carbamino formation

o 30%: due to ↑buffering (imidazole pKa 6.6 -> 8.2)

Isohydric buffering

·   For each mol O2 unloaded by Hb, 0.7mol H+ can be added without ↓ plasma pH

·   Due to Haldane effect

 

 

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