Oxygen dissociation curve with graph and factors that shifts curve to left and right*
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Oxygen Dissociation Curve (ODC)
The Curve
The ODC describes the relationship between the partial pressure of oxygen (PO₂) and the percent saturation of hemoglobin (SaO₂). It has a characteristic sigmoid (S-shaped) form — not hyperbolic — because hemoglobin exhibits cooperative binding: once one heme group binds O₂, subsequent binding to the remaining three heme groups becomes progressively easier (T → R quaternary conformational shift).
Key Reference Points
| Point | Value | Significance |
|---|---|---|
| P₅₀ | ~26–27 mmHg | PO₂ at which Hb is 50% saturated (standard conditions) |
| Arterial PO₂ | ~95–100 mmHg | Hb ~97–99% saturated |
| Venous PO₂ | ~40 mmHg | Hb ~75% saturated → only ¼ of O₂ delivered at rest |
The flat upper portion (PO₂ 60–100 mmHg) means lung disease causing modest drops in alveolar PO₂ still allows near-normal O₂ loading. The steep middle portion (PO₂ 20–60 mmHg) enables large amounts of O₂ to be released to tissues with relatively small drops in PO₂ — maximizing tissue delivery.
Shifts of the Curve
A right shift = ↑ P₅₀ = decreased O₂ affinity → more O₂ unloaded to tissues
A left shift = ↓ P₅₀ = increased O₂ affinity → less O₂ released to tissues
A left shift = ↓ P₅₀ = increased O₂ affinity → less O₂ released to tissues
RIGHT Shift (↓ Affinity, more O₂ delivered to tissues)
| Factor | Mechanism |
|---|---|
| ↓ pH (acidosis) | H⁺ stabilizes the T (deoxy) form of Hb — the Bohr effect |
| ↑ CO₂ | Direct carbamino binding + H⁺ generation via carbonic anhydrase → Bohr effect |
| ↑ Temperature | Weakens Hb–O₂ bonds; seen during exercise, fever, sepsis |
| ↑ 2,3-DPG (2,3-BPG) | Binds deoxy-Hb β-chains → stabilizes T form; ↑ in chronic hypoxia, anemia, high altitude |
Clinical scenario — exercise: Active muscles release CO₂ and acids, raise temperature → right shift → O₂ offloaded at PO₂ as high as 40 mmHg even when 70% already extracted. Lungs then see a reverse shift, loading more O₂.
LEFT Shift (↑ Affinity, less O₂ delivered to tissues)
| Factor | Mechanism |
|---|---|
| ↑ pH (alkalosis) | Stabilizes R (oxy) form; reverse Bohr effect |
| ↓ CO₂ (hypocapnia) | Less H⁺ generated; Hb holds O₂ |
| ↓ Temperature | Strengthens Hb–O₂ bonds (e.g., hypothermia) |
| ↓ 2,3-DPG | Stored bank blood (DPG degrades in 24 h), neonates |
| Carbon monoxide (CO) | CO has 240× greater affinity for Hb; forms carboxyhemoglobin AND causes conformational change → left shift of remaining Hb = double toxicity |
| Fetal hemoglobin (HbF, α₂γ₂) | γ-chains interact weakly with 2,3-DPG → inherently higher O₂ affinity than HbA; facilitates O₂ transfer from maternal to fetal circulation across placenta |
| Methemoglobin | Fe²⁺ → Fe³⁺; non-O₂-carrying subunits increase affinity of remaining subunits → left shift |
| High altitude (acutely) | Respiratory alkalosis from hyperventilation → left shift initially (later offset by ↑ 2,3-DPG) |
Summary Mnemonic
Right shift = CADET (CO₂↑, Altitude↓ O₂ affinity, DPG↑, Exercise, Temperature↑)
Left shift = opposite of each + CO, HbF, MetHb
Left shift = opposite of each + CO, HbF, MetHb
Sources:
- Fishman's Pulmonary Diseases and Disorders — Oxygen Dissociation Curve
- Guyton & Hall Textbook of Medical Physiology — Bohr Effect & BPG
- Lippincott Biochemistry, 8th ed. — Bohr Effect mechanism
- Mulholland & Greenfield's Surgery, 7e — Table 10.6 Factors Altering ODC
- Harrison's Principles of Internal Medicine, 21st ed. — ODC figure
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