Oxygen Transport & Dissociation Curve

🩸 Oxygen Transport & Dissociation Curve

Haemoglobin oxygen carriage, the oxyhaemoglobin dissociation curve, right and left shifts, carbon monoxide poisoning, and 2,3-DPG — the essential O₂ physiology for MRCS.

Oxygen Transport — How O₂ Is Carried in Blood

Oxygen is transported in blood in two forms: (1) bound to haemoglobin (97–98% of total) and (2) dissolved in plasma (2–3%). The dissolved fraction follows Henry’s Law (proportional to PaO₂). The haemoglobin-bound fraction is the critical reservoir.

🧠 Oxygen Content Equation — CaO₂

CaO₂ = (Hb × 1.34 × SaO₂) + (PaO₂ × 0.0225)

Where:
Hb = haemoglobin concentration (g/dL)
1.34 = Hüfner’s constant (mL O₂ per gram Hb at full saturation)
SaO₂ = arterial O₂ saturation (fraction, e.g. 0.98)
PaO₂ = arterial partial pressure of O₂ (kPa) × 0.0225 = dissolved component

Normal example: CaO₂ = (15 × 1.34 × 0.98) + (13.3 × 0.0225)
= 19.7 + 0.3 = ~20 mL O₂/dL blood

Key insight: The dissolved component (0.3 mL/dL) is tiny compared to Hb-bound (19.7 mL/dL). This is why anaemia (↓ Hb) profoundly reduces O₂ delivery despite normal SpO₂.

Oxygen Delivery (DO₂) and Consumption (VO₂)

Parameter	Formula	Normal Value	Clinical Notes
O₂ Delivery (DO₂)	DO₂ = CO × CaO₂ × 10	~1000 mL/min	Product of cardiac output and O₂ content. (×10 converts dL to mL per min per 5L CO). DO₂ falls with: anaemia (↓ Hb), hypoxaemia (↓ SaO₂), or low CO.
O₂ Consumption (VO₂)	VO₂ = CO × (CaO₂ − CvO₂) × 10	~250 mL/min	Fick principle. Normal extraction ratio = VO₂/DO₂ = 25%. Can increase to ~75% during exercise/shock (maximal extraction).
O₂ Extraction Ratio (OER)	OER = VO₂/DO₂ = (SaO₂ − SvO₂)/SaO₂	~25%	Normal tissues extract ~25% of delivered O₂. In shock: ↑ OER (tissues extract more). SvO₂ falls: normal 65–75%, critically low <50%.
Critical DO₂	~330 mL/min	Threshold	Below this DO₂, extraction cannot compensate → VO₂ becomes supply-dependent → tissue hypoxia → lactate production. Critical DO₂ is the threshold where anaerobic metabolism begins.

Hüfner’s constant

1.34 mL O₂/g Hb

Theoretical max is 1.39 — real-world is 1.34 due to metHb and COHb

Normal CaO₂

~20 mL/dL

Predominantly Hb-bound. Dissolved = ~0.3 mL/dL.

Normal DO₂

~1000 mL/min

CO (5 L/min) × CaO₂ (20 mL/dL) × 10

Normal VO₂

~250 mL/min

Resting. ↑ to 3500 mL/min in maximal exercise.

Mixed venous SvO₂

65–75%

Reflects global O₂ utilisation. <50% = inadequate DO₂.

P50

26.6 mmHg (3.5 kPa)

PO₂ at which Hb is 50% saturated. Index of Hb-O₂ affinity.

Why Anaemia is More Dangerous than Hypoxaemia for O₂ Delivery

Anaemia vs Hypoxaemia — The O₂ Content Comparison

Using the O₂ content equation: a patient with normal Hb (15 g/dL) but SpO₂ 90% (PaO₂ ~60 mmHg) has CaO₂ = 15 × 1.34 × 0.90 = 18.1 mL/dL. A patient with normal SpO₂ (98%) but severe anaemia (Hb 7 g/dL) has CaO₂ = 7 × 1.34 × 0.98 = 9.2 mL/dL. The anaemic patient has less than HALF the O₂ content despite a normal SpO₂ reading. SpO₂ is NOT a surrogate for O₂ delivery — haemoglobin concentration is equally critical. A blood transfusion (raising Hb 7→10) improves DO₂ far more than raising FiO₂ in a patient with normal SpO₂.

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