Respiratory - Carbon Dioxide Transport

Question 1

What is the rate of carbon dioxide production in a 70 kg adult male

Answer 1

200 ml/min at standard temperature and pressure.

Vigorous exercise: CO2 production as high as 4000ml/min

Question 2

Compared the total volume of O2 stored in the body versus CO2

Answer 2

O2 –> 1.5L

• 850 mls in RBC
• 250 mls in myoglobin
• 450 mls in FRC

CO2 –> 120 L stored throughout the body in various forms

Question 3

How is Carbon Dioxide transported in the circulation

Answer 3

1. Dissolved in plasma
   - more vs O2 as solubility co-efficient is 20 x higher
2. Carbamino compounds (Hb and proteins)
   - Carbaminohaemoglobin
   - This is formed when CO2 reacts with terminal amine group within the Hb molecule
3. As bicabonate

Question 4

What is Henry’s law with regards to CO2 transport in blood

Answer 4

The volume of CO2 dissolved in plasma is directly proportional to the partial pressure of CO2 above it.

Question 5

Which forms carbaminohaemoglobin more readily: Oxyhaemoglobin or deoxyhaemoglobin and what is this effect called?

Answer 5

Deoxyhaemoglobin –> The Haldane effect

Logical –> Binding of CO2 more NB in tissues (where deoxyHb formed) and Release of CO2 more NB in lung where oxyHb formed.

Question 6

Where is Carbonic Anhydrase found RBC or plasma or both and why

Answer 6

Found in RBCs and not in plasma.

Carbonic Anhydrase catalyses the following equilibrium reaction

H20 + CO2 H2CO3H+ + HCO3-

H2O / CO2 / HCO3- are able to traverse the cell membrane
H+ cannot traverse the cell membrane

CHLORIDE SHIFT
H+ BINDING TO HISTIDINE RESIDUES
keeps the concentration of H+ and HCO3 low hence the reaction can continue and CO2 continues to be converted to HCO3- for transport and storage

Question 7

H20 + CO2 H2CO3H+ + HCO3-

If H+ and HCO3- were to accumulate in the RBC then the equilibrium reaction would stop. What mechanisms exist to prevent this from happening

Answer 7

1. CHLORIDE SHIFT (HAMBURGER EFFECT)
   - HCO3- diffuses out into the plasma down conc. gradient.
   - It is exchanged for a Cl- ion to maintain electrical neutrality (Cl-/HCO3- exchanger)
2. H+ BINDS TO HISTIDINE RESIDUES
   - H+ intracellular hence reduced
   - DeoxyHb is able to bind H+ better than oxyHb (Haldane effect)

Question 8

Define the Haldane effect

Answer 8

The Haldane effect is the observation that deoxyHb is a more effective net carrier of CO2 than is oxyHb.

1. DeoxyHb more readily forms carbamino compounds
2. DeoxyHb is a better H+ acceptor than oxyHb (allowing increased HCO3- formation)

Therefore the metabolic waste products are efficiently transported away from tissues to the lungs.

Question 9

Define the Bohr effect

Answer 9

The Bohr effect describes the finding tht increased CO2 tension or reduced pH shifts the P50 of the OHDC to the right therefore resulting in Hb having a lower binding affinity for O2. Therefore additional O2 is offloaded to more metabolically active tissues.

Question 10

What proportion of carbon monoxide is transported in each form in arterial and venous blood

Answer 10

ARTERIAL
Dissolved 5%
Carbamino 5%
Bicarbonate 90%

VENOUS (more deoxyHb therefore more carbamino compounds)

Dissolved 10%
Carbamino 30%
Bicarbonate 60%

Question 11

Draw 1) the CO2 dissociation curve and 2) the proportions of CO2 transport forms in arterial blood curve

Answer 11

Page 38 Chambers

Question 12

Which graph is a graphical demonstration of the Haldane effect –> draw it

Answer 12

The CO2 content vs PaCO2 graph for venous and arterial blood. Page 38 Chambers to draw.

Question 13

If a healthy patient stops breathing (for example on induction of anaesthesia), how quickly does CO2 rise and O2 fall

Answer 13

O2 consumption = 250ml/min
CO2 production = 200 ml/min

PaCO2 rises by 0.6 kPa/min

O2 depends on a number of variables

1. Hb concentration
2. FRC volume
3. Total Blood volume

Typically SaO2 falls to 70% (PaO2 5 kPa) after 2 minutes

Completely denitrogenated FRC –> ±6 minutes until SaO2 starts to fall.