Respiratory Physiology Part D: CO2 Transport

Question 1

How is CO2 carried?

Answer 1

1. Dissolved in Plasma (8%)
2. Bound to Hemoglobin = carbamino Hb (CO2 on globin) = 20%
3. As bicarbonate ions (HCO3-) = 72%

Question 2

CO2 dissolved in plasma at the lungs

Answer 2

• Alveolar PCO2 = 40 mmHg
• Resting Arterial PCO2 = 45 mmHg
• Venous PCO2 = 40 mmHg
• CO2 diffuses: capillary alveolus

Question 3

CO2 dissolved in plasma at the tissues

Answer 3

• Arterial PCO2 = 40 mmHg
• ICF PCO2 = > 45 mmHg
• ISF PCO2 = 45 mmHg
• Resting Venous PCO2 = 45 mmHg
• CO2 diffuses: cell ISF capillary

Question 4

CO2 bound to hemoglobin

Answer 4

CO2 binds to deoxyHb better than to oxyHb ∴ Hb binds CO2 readily at the tissues

Question 5

CO2 as bicarbonate ions inside RBC at tissues (increased CO2)

Answer 5

- CO2 + H2O   H2CO3   H+ + HCO3-
Carbonic anhydrase in RBC
- H+  +  Hb    HbH (Hb is a buffer)
- HCO3- transported out of rbc in exchange for Cl- = chloride shift (allows more HCO3- to be made)
∴ venous blood rbc’s have more Cl-

Question 6

CO2 as bicarbonate ions inside RBC at lungs

Answer 6

1. O2 + deoxyHb HbO2
   - Note: deoxyHb = HbH and/or HbCO2
   - HbO2 binds CO2 + H+ poorly ∴ CO2 released (= Haldane effect) + H+ released
2. H+ + HCO3- H2CO3 CO2 + H2O
   Carbonic Anhydrase
   ∴ ⇓ HCO3- in RBC ∴ HCO3- moves into rbc (down gradient) - exchanges for Cl- ( = reverse chloride shift)
   -Note: CO2 moves from rbc ⇒ plasma ⇒ alveolar air ⇒ out