16. Oxygen Dissociation Curve & Associated Gas Transport Effects

Oxygen Dissociation Curve Overview

• Graph plots percentage saturation of hemoglobin (Hb) on the y-axis against partial pressure of oxygen ((P{O2})) on the x-axis.
• Characteristic sigmoidal (S-shaped) profile reflects cooperative binding of O(_2) to Hb.
• Region-specific behavior:
  • Steep middle section ((P{O2}\approx 20\text{–}40\,\text{mmHg})): small changes in (P{O2}) produce large changes in saturation, facilitating O(_2) unloading in tissues.
  • Plateau at high (P{O2}) ((\ge 80\,\text{mmHg})): ensures near-maximal loading in lungs even if alveolar (P{O2}) falls moderately (e.g., mild altitude).

Shape & Mechanics (Cooperative Binding)

• Each Hb molecule possesses 4 heme groups → can bind 4 O(_2) molecules.
• Binding of the first O(_2) induces a conformational change from T-state (tense) to R-state (relaxed), increasing affinity of remaining sites — underpinning the sigmoid curve.
• Mathematical representation often modeled with the Hill equation:
  ${\text{Saturation}} = \frac{(P<em>{O</em>2})^n}{P<em>{50}^n + (P</em>{O<em>2})^n}$ where (n) ≈ 2.8–3 (Hill coefficient) and (P{50}) ≈ 26–27 mmHg for adult Hb.

Bohr Effect ((\uparrow CO_2) / (\downarrow pH) → (\rightarrow) Right Shift)

• Definition: Rise in CO(2) or H(^+) concentration reduces Hb-O(2) affinity, shifting the curve rightward.
• Molecular basis: H(^+) protonates specific amino acids (e.g., histidines) stabilizing the T-state.
• Physiological significance:
  • Actively metabolizing tissues generate CO(2) and H(^+) → Hb unloads more O(2).
  • Quantitatively, for every decrease of 0.1 pH unit, (P_{50}) can increase by ≈ 3–4 mmHg.
• Example: During intense exercise, muscle pH may fall toward $pH\approx 7.2$, ensuring enhanced O(_2) delivery.

Haldane Effect ((\downarrow O2) → (\uparrow CO2) Transport)

• Opposite emphasis of Bohr: Describes how deoxygenated Hb carries more CO(_2) and H(^+) than oxygenated Hb.
• Sequence in systemic tissues:
  1. O(_2) leaves Hb → produces HHb (reduced Hb), a better proton acceptor.
  2. HHb buffers H(^+) generated from carbonic anhydrase reaction: $CO<em>2 + H</em>2O \leftrightarrow H<em>2CO</em>3 \leftrightarrow H^+ + HCO_3^-$
  3. Resulting shift favors continued conversion of CO(2) into bicarbonate, enhancing total CO(2) carriage.
• In the lungs, high (P{O2}) reverses the process: O(2) loads onto Hb, protons are released, driving bicarbonate back to CO(2) for exhalation.
• Net outcome: Approx. (\frac{1}{3}) of CO(_2) transport capacity is attributable to the Haldane effect.

Factors Shifting the Curve

• Right Shift ((\rightarrow)): promotes O(_2) unloading.
  • (\uparrow P{CO2})
  • (\downarrow pH) (Bohr effect)
  • (\uparrow T) (temperature)
  • (\uparrow 2,3\,\text{BPG})
  • High altitude adaptation: persistent right shift via 2,3-BPG rise counters reduced ambient (P{O2}).
• Left Shift ((\leftarrow)): enhances O(_2) loading.
  • (\downarrow P{CO2})
  • (\uparrow pH)
  • (\downarrow T)
  • Fetal Hb (HbF) — lower 2,3-BPG binding → higher affinity than adult Hb; crucial for maternal-fetal transfer.

Hemoglobin: Additional Functional Roles

• CO(_2) Transport Forms
  • Dissolved: $\approx 5\%$
  • Carbamino compounds (bound to globin): $\approx 5\%$; enhanced by Haldane effect.
  • Bicarbonate (major): $\approx 90\%$; facilitated by Hb buffering.
• Buffering Capacity
  • Imidazole side chains in Hb account for (\approx 60\%) of the blood’s non-bicarbonate buffering power.
• Clinical Relevance
  • Shifts inform ventilator settings, transfusion strategies, altitude acclimatization, and evaluation of acid-base disorders.
  • Carbon monoxide (CO) binding forms carboxyhemoglobin, producing a left-shift yet reduced O(2) carrying capacity → tissue hypoxia despite normal (P{O_2}) readings.