24A11: Exam Report
Compare and contrast the carriage of oxygen in the blood with the carriage of carbon dioxide
55% of candidates passed this question.
A good response included normal values of the partial pressures of both oxygen and carbon dioxide including the arterial and venous contents.
Methods of carriage (haemoglobin, dissolved and other forms), haemoglobin binding characterisitcs (co-operative binding and affinities); and impact on loading/off loading at tissues and lungs for both oxygen and carbon dioxide was included.
Correctly labelled diagrams with a brief accompanying explanation could be used to convey some of these concepts.
F8i / 24A11: Compare & contrast the carriage of O2 & CO2 in blood
Content of Arterial Blood
mL/dL
mmHg
CO2
48mL
40mmHg
O2
21mL
100mmHg
Content of Venous Blood
mL/dL
mmHg
CO2
52mL
46mmHg
O2
15mL
40mmHg
Mechanism of O2 Transport & ODC
CaO2 = Dissolved O2 + OxyHb
Dissolved O2
- Solubility co-efficient 0.003
- Obeys Henry’s Law → the amount carried dissolved in solution is directly ∝ to the partial pressure
- ∴pO2 x 0.003 = O2 mL/dL
Oxyhaemoglobin
- O2 binds reversibly to Hb: Hb + O2 ⮂ HbO2
- HUFFNER’S CONSTANT = 1.34mL O2 per 1g Hb
- O2 binds to heme portion of Hb
- 4 x heme → ∴1Hb can bind up to 4 O2
- Displays +ve COOPERATIVITY → affinity for O2 is lowest at first O2 binding because DeoxyHb Is in TENSE configuration
- With each subsequent O2 binding heme is ↑O2 affinity
ODC
- This is what gives ODC a sigmoid shape

- P50 = partial pressure oxygen in blood where Hb is 50% saturated with O2 → it is a measure of O2 affinity
- ODC can be shifted to give Hb altered affinity for O2 & this is compared with P50
R) Shift ODC
- ↑CO2
- ↑H+
- ↑Temp
- ↑2, 3 DPG
- ↓pH
= ↓affinity = offloads O2
L) Shift ODC
- ↓CO2
- ↓H+
- ↓Temp
- ↓2,3 DPG
- ↑pH
= ↑affinity = binds O2 tighter
CO2 Carriage in Blood & CO2 Dissociation Curve
Carbamino Compounds + Dissolved + HCO3–
Carbamino Compounds
- Carbamino compound = CO2 + terminal amine group of a protein (i.e. Hb)
- Hb most abundant protein → 15g/dL
- ∴its terminal amine group most important for CO2 carriage
- 4 terminal amine groups
- CO2 binds to α-chain
- DeoxyHb forms carbamino compounds 3.5 x more readily than OxyHb
NB: 70% HALDANE EFFECT → for any given PCO2, deoxygenated blood is able to carry more CO2
Dissolved
- Obeys HENRY’S LAW → the [ ] of a gas in liquid is ∝ to its partial pressure
- CO2 is x 20 more soluble cf. O2
- Solubility co-efficient 0.03
- ∴ Dissolved CO2 (mL) = 0.03 x pCO2
HCO3–

- Occurs in RBC
- HCO3– diffuses out, H+ cannot
- Cl – is exchanged for HCO3– to maintain electroneutrality
- DeoxyHb is less acidic cf. oxyHb
- Accepts H+ to drive equation forward → 30% HALDANE EFFECT
CO2 Dissociation Curve

- CO2 is much more linear
- Lacks Positive Cooperativity
- Steeper because of greater content per mL
- HALDANE EFFECT responsible for A – V difference