O₂ and CO₂ Transport in Blood
Key facts on haemoglobin's oxygen dissociation curve, its shifters, and the three forms of CO₂ carriage.
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Questions Covered in This Set
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How much O₂ does 1 g of haemoglobin bind, and what is normal arterial O₂ content?
1.34 mL O₂ per gram Hb; with 15 g/dL Hb this gives ~20 mL/dL, versus only ~0.3 mL/dL dissolved.
Write the equation for arterial oxygen content (CaO₂).
CaO₂ = (1.34 × Hb × SaO₂) + (0.003 × PaO₂)
State the three landmark points of the oxyhaemoglobin dissociation curve.
PO₂ 100 mmHg → ~98% (arterial); PO₂ 60 mmHg → ~90% (the 'cliff edge'); PO₂ 40 mmHg → ~75% (mixed venous).
Why is the oxyhaemoglobin curve sigmoid rather than hyperbolic?
Cooperativity — binding of O₂ to one of the four subunits shifts the tetramer from the low-affinity T (tense) state toward the high-affinity R (relaxed) state.
What is P₅₀ and its normal value?
The PO₂ at which Hb is 50% saturated; normally 26–27 mmHg. Higher P₅₀ = right shift = lower affinity.
Give the mnemonic for right shift of the curve.
CADET, face Right! — CO₂, Acid (↓pH), 2,3-DPG, Exercise, Temperature. All ease unloading in active tissue.
What causes left shifts of the curve?
↓temperature, ↓CO₂, alkalosis, ↓2,3-DPG, carboxyhaemoglobin, methaemoglobin, and fetal haemoglobin (P₅₀ ~19 mmHg).
Define the Bohr effect.
↑H⁺ (↓pH) binds haemoglobin and stabilises the T state, reducing O₂ affinity and promoting unloading in acidic tissues.
List the three forms of CO₂ carriage and their approximate fractions.
Bicarbonate ~70%, carbamino compounds ~23%, dissolved ~7%.
What is the chloride shift?
HCO₃⁻ formed in the red cell (via carbonic anhydrase) exits into plasma in exchange for Cl⁻ via band 3/AE1; H⁺ is buffered by haemoglobin.
Define the Haldane effect.
Deoxygenated haemoglobin carries more CO₂ (as carbamino and by buffering H⁺) than oxygenated Hb — the mirror image of the Bohr effect.
Why is carbon monoxide poisoning so dangerous?
It slashes O₂ content and left-shifts the curve so carried O₂ isn't released, while PaO₂ and pulse oximetry read falsely normal.
