Oxygen Transport & Hemoglobin–Oxygen Dissociation

Context & Scope

• Lecture follows units on pulmonary ventilation and external/internal respiration.
• Current focus: HOW oxygen (and later other gases) are transported within the bloodstream once they have crossed the respiratory membrane.
• Sequence of movement:
  1. Alveoli → pulmonary capillary blood (external respiration)
  2. Pulmonary veins → left side of heart → systemic arteries
  3. Systemic capillaries → tissue cells (internal respiration)
  4. De-oxygenated blood → right side of heart → lungs for re-oxygenation.

Oxygen Transport: Two Compartments

• Oxygen has very low water solubility ⇒ cannot be carried efficiently in plasma alone.
• Distribution of total arterial $\text{O}_2$ content:
  • $1.5\,\%$ dissolved directly in plasma (the ONLY portion that can diffuse into cells).
  • $98.5\,\%$ reversibly bound to the transport protein hemoglobin (Hb) inside red blood cells (RBCs).

Hemoglobin (Hb) Structure & Multi-Ligand Binding

• Macromolecule located exclusively in RBC cytoplasm.
• Quaternary structure: 4 polypeptide subunits (globins)
  • 2 α-chains + 2 β-chains.
• Each subunit contains a prosthetic heme group with a centrally chelated Fe²⁺ ion → binding site for one $\text{O}_2$ molecule.
• Maximum carrying capacity: 4 $\text{O}_2$ per Hb (full saturation).
• Other reversible ligands (bind at globin, not heme)
  • $\text{CO}_2$
  • $\text{H}^+$ (protons)
  • 2,3-bisphosphoglycerate (2,3-BPG)
• Core reversible reaction (oxygenation/de-oxygenation): $\text{Hb} + \text{O}<em>2 \rightleftharpoons \text{HbO}</em>2$
  • Pulmonary capillaries: reaction driven → right (loading).
  • Systemic capillaries: reaction driven → left (unloading).

Percent Saturation Terminology

• "Percent saturation" (SaO₂, $\text{O}_2$ sat, O₂ Sats) = % of all available heme Fe²⁺ sites occupied by oxygen.
  • Fully saturated Hb = 4 $\text{O}_2$ per molecule.
  • Clinically normal: $\ge 95\,\%$.
  • < $90\,\%$ signals hypoxemia / pathology.

Primary Determinant: Partial Pressure of Oxygen ($P<em>{\text{O}</em>2}$)

• Law: ↑ $P<em>{\text{O}</em>2}$ ⇒ ↑ Hb affinity and ↑ saturation.
• Oxygen-Hemoglobin Dissociation (or Saturation) Curve
  • X-axis: $P<em>{\text{O}</em>2}$ (mm Hg)
  • Y-axis: % Hb saturation
  • S-shaped (sigmoidal) due to cooperative binding among the 4 heme sites.
• Canonical reference points:
  1. Pulmonary capillaries / arterial blood
  • $P<em>{\text{O}</em>2}=100\,\text{mmHg}$ ⇒ ~$98\,\%$ saturation (oxygen loading).
  1. Systemic capillaries at REST
  • $P<em>{\text{O}</em>2}=40\,\text{mmHg}$ ⇒ ~$75\,\%$ saturation.
  • Oxygen delivered = $98\% - 75\% = 23\%$ of Hb-carried O₂.
  • Remaining $75\%$ = OXYGEN RESERVE (held for periods of ↑ demand).
  1. Contracting skeletal muscle during EXERCISE
  • $P<em>{\text{O}</em>2}\approx20\,\text{mmHg}$ ⇒ ~$35\,\%$ saturation.
  • Oxygen delivered = $98\% - 35\% = 63\%$ (utilisation of the reserve).

Metabolic Need & The Five Modifiers ("CADET face RIGHT")

Any INCREASE in the following factors shifts the dissociation curve to the RIGHT, thereby
↓ Hb–O₂ affinity and ↑ oxygen unloading.
Mnemonic: "CADET face right" or simply "Right Release".

1. $P<em>{\text{CO}</em>2}$ (Carbon Dioxide)

• CO₂ binds globin, destabilising oxy-Hb.
• ↑ $P<em>{\text{CO}</em>2}$ typical of active tissues producing more metabolic CO₂.
• Example at $P<em>{\text{O}</em>2}=30\,\text{mmHg}$:
  • Normal $P<em>{\text{CO}</em>2}$ → 50 % saturation.
  • High $P<em>{\text{CO}</em>2}$ → 35 % saturation (65 % unloaded).
  • Low $P<em>{\text{CO}</em>2}$ → 65 % saturation (35 % unloaded).

2. Acidity / $[\text{H}^+]$ (pH)

• Mechanistically linked to CO₂ via carbonic-acid reaction (see Bohr Effect below).
• ↑ $[\text{H}^+]$ (↓ pH) ⇒ right shift.
• Quantitative example (again at $P<em>{\text{O}</em>2}=30\,\text{mmHg}$):
  • High $[\text{H}^+]$ → ~35 % saturation.
  • Low $[\text{H}^+]$ → ~65 % saturation.

3. 2,3-Bisphosphoglycerate (2,3-BPG, aka Diphosphoglycerate)

• Metabolic by-product of RBC glycolysis.
• Binds deoxy-Hb, stabilising the low-affinity state and pushing O₂ off.
• ↑ whenever RBC glycolytic rate rises (e.g., chronic hypoxia, anemia, high altitude).

4. Exercise

• Composite factor because exercise simultaneously ↑ $P<em>{\text{CO}</em>2}$, ↑ $[\text{H}^+]$, ↑ temperature, and ↑ 2,3-BPG.
• Dramatically boosts O₂ delivery (63 % unloaded vs 23 % at rest).

5. Temperature

• Heat is a universal by-product of metabolism.
• ↑ T disrupts Hb-O₂ binding; ↓ T has opposite effect.
• Data at $P<em>{\text{O}</em>2}=30\,\text{mmHg}$:
  • High T → 25 % saturation (75 % unloaded).
  • Low T → 82 % saturation (18 % unloaded).

The Bohr Effect (CO₂ & pH Interdependence)

• Fundamental equation to memorise: $\text{CO}<em>2 + \text{H}</em>2\text{O} \rightleftharpoons \text{H}<em>2\text{CO}</em>3 \rightleftharpoons \text{HCO}_3^- + \text{H}^+$
  • Catalysed by carbonic anhydrase inside RBCs.
• Consequences:
  • ↑ tissue CO₂ → ↑ $\text{H}^+$ → ↓ pH.
  • This proton load allosterically lowers Hb affinity, enhancing O₂ release (right shift).
• Definition: Bohr Effect = Decrease in Hb-O₂ saturation produced by ↓ pH or ↑ CO₂.

Overall Metabolic Rationale

• Cellular respiration summary (master equation to recall):
  $\text{Nutrients} + \text{O}<em>2 \rightarrow \text{CO}</em>2 + \text{H}_2\text{O} + \text{ATP} + \text{heat}$
• Any signal of ↑ right-hand products (CO₂, H₂O heat) or intermediates (2,3-BPG) informs Hb that the locale is metabolically active and needs more O₂.
• Homeostatic outcome: targeted oxygen delivery without needless global changes in arterial $P<em>{\text{O}</em>2}$.

Key Clinical & Physiological Take-Aways

• Saturation monitors (pulse oximeters) report %SaO₂; values < $90\,\%$ are red flags.
• Oxygen reserve (~75 % at rest) provides safety margin for sudden exertion or hypoxic stress.
• Rightward curve shifts (CADET) are adaptive but can exacerbate hypoxemia if arterial $P<em>{\text{O}</em>2}$ is already low.
• Leftward shifts (opp. of CADET) occur with ↓ CO₂, ↓ H⁺, ↓ temperature, ↓ 2,3-BPG—e.g., in fetal Hb or during certain transfusions—and enhance loading at the expense of unloading.

Memory Aids Recap

• “CADET face RIGHT” → CO₂, Acidity, DPG, Exercise, Temperature cause right shift.
• “Right Release” → right-shifted curve means oxygen RELEASE from Hb.