5. Carbon Dioxide Transport

Question 1

Normal CO2 values

Answer 1

Carbon dioxide (CO2)
(MW 44; BP –79 °C;
critical temperature 31 °C)

is one of the main end products of metabolism.
The body contains approximately

120 L of CO2.

> PiCO2 (inspired) 0.03 kPa

> PECO2 (expired) 4 kPa

> PaCO2 (arterial) 5.3 kPa

> PACO2 (alveolar) 5.3 kPa [CO2 content 21.9 mmol/L]

> PVCO2 (venous) 6.1 kPa [CO2 content 23.7 mmol/L]

Question 2

How is CO2 transported from the

cells to the lungs?

Answer 2

Under resting conditions
CO2 production in the body
is approximately 200 mL/min.

The CO2 formed in the cells
diffuses through the interstitial
space to enter the venous circulation.

CO2 is transported in the blood in three forms:

> 5% dissolved

(CO2 is 20 times more soluble in blood than O2)

> 5% carbamino compounds
(combined with NH2 groups on haemoglobin)

> 90% bicarbonate (mainly in plasma).

                 CA CO2 +H2O    H2CO3  H+ +HCO3

Question 3

Describe the events that take place between the tissue cells and RBCs.

Answer 3

> Reaction between CO2 and H2O
is slow in the plasma but fast

(×1000 faster)

within the red blood cell (RBC)

due to the intracellular presence of
the enzyme carbonic anhydrase (CA).

Question 4

Draw a schematic rep of the shift in the cell

Answer 4

pg 16

Question 5

Compare the CO2 dissociation curve with HbO2 dissociation curve.

Answer 5

CO2 dissociation curve is influenced

by the state of oxygenation of the Hb

(Haldane effect)

where oxyhaemoglobin carries less CO2 than
deoxyhaemoglobin for the same PCO2.
CO2 dissociation curve is more linear than the oxyhaemoglobin dissociation
curve which is sigmoid in shape.

Question 6

Draw the physiological

dissociation curve for CO2.

Answer 6

pg 17

Question 7

Chloride shift?

Answer 7

> HCO3

– formed in the above reaction
diffuses out of the RBC.

However, the accompanying H+ ion
cannot follow due to the relative impermeability

of the red cell membrane to such cations.

In order to maintain electrical neutrality,

Cl– ions diffuse into the
red cell from the plasma,
the ‘chloride shift’.

Question 8

Haldane and Bohr effects

Answer 8

> Haldane effect describes
how CO2 transport is affected
by the state of oxygenation of Hb.

This is because deoxyhaemoglobin
is better than oxyhaemoglobin in:

• Combining with CO2 to form
carbamino compounds

(in turn assisting the blood
to load more CO2 from tissues
for removal at the lungs)

• Combining with H+ ions
(in turn assisting the blood
to load more CO2
from the tissues)

> As CO2 leaves the tissue cells 
and enters the RBCs, 
it causes more O2 to
dissociate from Hb 
(Bohr shift) and thus more 
CO2 combines with 
Hb and more HCO3+ is produced.