Oxygen Dissociation Curve — DNB Comprehensive Answer

1. DEFINITION

2. KEY DIAGRAMS

Diagram 1 — Standard ODC (Total vs. Dissolved O₂)

• Red line = Total O₂ content (Hb-bound + dissolved)
• Blue line = Dissolved O₂ (linear, tiny proportion)
• P₅₀ = 26.5 mmHg (PO₂ at which Hb is 50% saturated)
• X-axis: PO₂ (mm Hg) | Y-axis: O₂ content (mL/100 mL blood)
• Flat upper portion (plateau): PO₂ 70–100 mmHg — loading zone (lungs)
• Steep middle portion: PO₂ 20–60 mmHg — unloading zone (tissues)

— Fishman's Pulmonary Diseases and Disorders, Fig. 15-1

Diagram 2 — Shifts of the ODC (Harrison's Principles)

• Blue solid line = Normal curve, P₅₀ at ~26 mmHg
• Green dashed = Left shift → ↑ O₂ affinity → Less O₂ delivered to tissues
• Purple dashed = Right shift → ↓ O₂ affinity → More O₂ delivered to tissues

Diagram 3 — Bohr Effect (pH shift, Lippincott Biochemistry)

• pH 7.2 → Rightward shift → Lower O₂ affinity → More O₂ released to tissues
• pH 7.6 → Leftward shift → Higher O₂ affinity → Less O₂ released

— Lippincott Illustrated Reviews: Biochemistry 8th Ed., Fig. 3.9

Diagram 4 — Effect of 2,3-BPG on ODC (Lippincott Biochemistry)

• Black curve (2,3-BPG = 0): Extremely high O₂ affinity, acts as O₂ trap
• Blue (normal, 5 mmol/L): Standard ODC
• Red (8 mmol/L, high altitude): Rightward shift, ↑ O₂ delivery to tissues

— Lippincott Illustrated Reviews: Biochemistry 8th Ed., Fig. 3.12

Diagram 5 — Multiple Shift Factors (Guyton & Hall)

1. ↑ Hydrogen ions (↓ pH)
2. ↑ CO₂
3. ↑ Temperature
4. ↑ 2,3-BPG

— Guyton & Hall Textbook of Medical Physiology, Fig. 41.10

3. SHAPE OF THE CURVE — WHY SIGMOID?

• Myoglobin has a hyperbolic curve (single polypeptide, single heme) — binds O₂ with uniform affinity; would not release O₂ at tissue PO₂
• Hemoglobin has a sigmoid (S-shaped) curve due to cooperative binding (allostery)
  • Hb is a tetramer (2α + 2β chains); each subunit influences the others
  • Binding of one O₂ molecule to a heme group increases affinity of remaining heme groups for O₂ — this is positive cooperativity
  • Hemoglobin's affinity for the last O₂ is ~300× greater than for the first
  • The structural basis: T (tense/deoxy) state has low O₂ affinity → O₂ binding converts to R (relaxed/oxy) state with high O₂ affinity

— Lippincott Illustrated Reviews: Biochemistry 8th Ed., pp. 100–101

4. PHYSIOLOGICAL SIGNIFICANCE OF SIGMOID SHAPE

• Even if PaO₂ falls from 100 to 70 mmHg (e.g., in mild lung disease), SaO₂ remains ~94% — significant protective reserve
• Below PaO₂ 60 mmHg (SaO₂ ~90%), the curve enters the steep zone — small falls in PaO₂ cause dramatic falls in SaO₂ → clinical hypoxia

5. P₅₀ — DEFINITION AND SIGNIFICANCE

• P₅₀ = PO₂ at which hemoglobin is exactly 50% saturated with oxygen
• Normal P₅₀ = 26.5 mmHg (at pH 7.4, 37°C, PaCO₂ 40 mmHg)
• P₅₀ is an index of oxygen affinity:
  • ↑ P₅₀ → Right shift → ↓ O₂ affinity → ↑ O₂ delivery to tissues
  • ↓ P₅₀ → Left shift → ↑ O₂ affinity → ↓ O₂ delivery (O₂ held tightly by Hb)

6. ALLOSTERIC EFFECTORS — FACTORS SHIFTING THE ODC

6A. Bohr Effect (H⁺ and CO₂)

• In metabolizing tissues, CO₂ is produced → enters RBCs → converted to H₂CO₃ (by carbonic anhydrase) → dissociates to HCO₃⁻ + H⁺
• ↑ H⁺ protonates specific histidine residues on deoxyhemoglobin → forms salt bridges → stabilizes T (deoxy) state → releases O₂
• The reaction:

HbO₂ + H⁺  ⇌  HbH⁺ + O₂

• In lungs: CO₂ is exhaled → ↓ pCO₂ → ↑ pH → shifts curve LEFT → Hb loads O₂
• In tissues: ↑ CO₂, ↓ pH → shifts curve RIGHT → Hb unloads O₂

6B. 2,3-Bisphosphoglycerate (2,3-BPG / 2,3-DPG)

• Most abundant organic phosphate in RBCs
• Synthesized via Rapoport–Luebering shunt of glycolysis
• Binds exclusively to deoxyhemoglobin (not oxyhemoglobin), in a central cavity formed by the two β-chains
• Binding stabilizes T (deoxy) state → ↓ O₂ affinity → RIGHT shift

• Chronic hypoxia (COPD, emphysema)
• High altitude (adaptation mechanism)
• Chronic anemia
• Pregnancy (facilitates O₂ transfer to fetus)
• Hyperthyroidism, fever

• Stored bank blood (O₂ affinity increases → stored blood = "O₂ trap")
  • 2,3-BPG restores in 6–24 hours after transfusion
• Septic shock, hypothyroidism, hexokinase deficiency

— Lippincott Illustrated Reviews: Biochemistry 8th Ed., pp. 107–108

6C. Temperature

6D. Carbon Monoxide (CO)

• CO binds hemoglobin iron with 220× greater affinity than O₂ → forms carboxyhemoglobin
• CO shifts remaining heme sites to R (oxy) conformation → LEFT shift of ODC
• Changes curve shape from sigmoid → hyperbolic
• Net result: Hb cannot release O₂ to tissues → tissue hypoxia despite normal PaO₂
• Treatment: 100% O₂ (or hyperbaric O₂) to displace CO

7. COMPREHENSIVE SHIFT SUMMARY TABLE

— Mulholland & Greenfield's Surgery 7e, Table 10.6; Lippincott Biochemistry 8e

8. FETAL HEMOGLOBIN (HbF)

• Structure: α₂γ₂ (γ-chains replace β-chains)
• γ-chains bind 2,3-BPG less avidly than β-chains → HbF has higher O₂ affinity than HbA (adult Hb)
• HbF ODC is shifted LEFT relative to maternal Hb
• Physiological purpose: Enables O₂ transfer from maternal blood (lower saturation) to fetal blood across placenta
• P₅₀ of HbF ≈ 19–20 mmHg vs. 26.5 mmHg for HbA
• After birth: HbF → HbA replacement over 3–6 months; compensated by ↑ 2,3-BPG to maintain O₂ delivery

— Mulholland & Greenfield's Surgery 7e; Morgan & Mikhail's Clinical Anesthesiology 7e

9. OXYGEN CONTENT EQUATION

• 1.34 mL = O₂ carrying capacity per gram of Hb (Hüfner's constant)
• 0.003 mL/dL/mmHg = solubility coefficient of dissolved O₂
• Normal CaO₂ ≈ 20 mL/100 mL blood
• Normal CvO₂ (mixed venous) ≈ 15 mL/100 mL blood (SvO₂ ~75%)
• O₂ delivery (DO₂) = CaO₂ × Cardiac Output
• O₂ extraction = CaO₂ − CvO₂ ≈ 5 mL/100 mL at rest (25% of delivered O₂)

10. FLOWCHART — O₂ LOADING & UNLOADING

ALVEOLAR CAPILLARY
PO₂ = 100 mmHg → High PO₂ + High pH (CO₂ exhaled)
         ↓
    Hb shifts to R (oxy) state
         ↓
    Hb LOADS O₂ (SaO₂ ~98%)
         ↓
ARTERIAL BLOOD → transported to tissues
         ↓
TISSUE CAPILLARY
↑ CO₂, ↑ H⁺, ↑ 2,3-BPG, ↑ Temperature
PO₂ = 40 mmHg (mixed venous)
         ↓
    Hb shifts to T (deoxy) state
    [Bohr Effect active]
         ↓
    Hb UNLOADS O₂ (~25% extracted at rest)
         ↓
VENOUS BLOOD (SvO₂ ~75%) → back to lungs

11. CLINICAL APPLICATIONS

12. COMPARISON: MYOGLOBIN vs. HEMOGLOBIN ODC

13. QUICK-RECALL BULLET POINTS

• Normal SaO₂ in arterial blood: 97–99% (PaO₂ ~100 mmHg)
• Normal SvO₂ in mixed venous blood: ~75% (PO₂ ~40 mmHg)
• P₅₀ normal: 26.5 mmHg; HbF P₅₀: ~19–20 mmHg (LEFT shifted)
• Hb carries ~1.34 mL O₂/g (Hüfner's constant); each gram Hb = theoretical max 1.39 mL
• At PaO₂ = 60 mmHg → SaO₂ ≈ 90% → critical threshold (steep slope begins)
• Bohr effect = rightward shift due to ↑ H⁺/CO₂ → enhances tissue O₂ delivery
• 2,3-BPG binds β-chains of deoxyhemoglobin → stabilizes T state → right shift
• HbF has γ-chains that bind 2,3-BPG poorly → LEFT shift → higher O₂ affinity
• CO poisoning → LEFT shift + converts sigmoid to hyperbolic curve
• Hill coefficient (n) for Hb ≈ 2.8 (reflects cooperativity; 1 = no cooperativity)
• Stored blood: 2,3-BPG depleted after days; replenished within 6–24 hours post-transfusion
• In exercise: RIGHT shift due to ↑ temp, ↓ pH, ↑ CO₂, ↑ 2,3-BPG → maximizes O₂ delivery
• Hypothermia in cardiac surgery deliberately uses LEFT shift → protects myocardium

14. POTENTIAL DNB VIVA / EXAM QUESTIONS

1. What is the oxygen dissociation curve? Why is it sigmoid and not hyperbolic?
2. Define P₅₀. What is its normal value? What does it indicate?
3. What is the Bohr effect? What is its physiological significance?
4. What is the oxygen-carrying capacity of hemoglobin? Write the Fick equation for oxygen content.

Sources: Fishman's Pulmonary Diseases and Disorders; Lippincott Illustrated Reviews: Biochemistry 8e; Guyton & Hall Textbook of Medical Physiology; Harrison's Principles of Internal Medicine 21e; Mulholland & Greenfield's Surgery 7e; Morgan & Mikhail's Clinical Anesthesiology 7e