Gas Exchange & Transport

Learning Outcomes

• Define partial pressure and relate it to a gas mixture (air).

• Explain the influence of partial pressure on gas transport by blood.

• Describe mechanisms of transporting O2 and CO2

• Discuss metabolic regulation of gas exchange (pH, temperature, BPG,PCO2,PO2)

• Evaluate effects of blood gases and pH on respiratory rhythm.

Fundamental Gas Laws

• Dalton’s Law

  • Total pressure of a gas mixture = sum of individual partial pressures.

  • Partial pressure ( P_x ) of a gas is directly proportional to its % composition.

• Henry’s Law

  • When a gas contacts a liquid, the quantity dissolved is proportional to its P_x.

  • At equilibrium, gas P_x is identical in gas & liquid phases.

  • Solubility modifiers:
    • \mathrm{CO2} ≈ 20× more soluble than \mathrm{O2}; \mathrm{N_2} barely dissolves.
    • ↑Temperature → ↓Solubility (gas escapes).

External vs. Internal Respiration

• External (pulmonary): diffusion across alveolar membrane.

• Internal (tissue): diffusion between systemic capillaries & cells.

• Both rely on:
  • Physical gas laws (partial pressure & solubility).
  • Alveolar gas composition.

Respiratory Membrane Characteristics

• Thickness: 0.5{-}1\,\mu\text{m}.

• Surface area: ≈40× total skin area.

• Pathologies:
  • Pulmonary edema → thicker barrier → ↓ diffusion.
  • Emphysema, tumors, inflammation, mucus → ↓ surface area.
  • Result: impaired gas exchange.

Partial-Pressure Gradients (Resting, Sea Level)

• \mathrm{O2}
  • Alveolar P{O2}=104\,\text{mmHg} vs. Pulmonary venous P{O_2}=40\,\text{mmHg}.
  • Steep gradient → rapid diffusion; equilibrium in ≈0.25\,\text{s} (1/3 capillary transit time).
  • Adequate even when cardiac output triples.

• \mathrm{CO2}
  • Venous P{CO2}=45\,\text{mmHg} vs. Alveolar P{CO2}=40\,\text{mmHg}. • Gradient less steep, but high solubility → equal exchange rate with \mathrm{O2}.

Ventilation–Perfusion (V̇/Q̇) Coupling

• Perfusion: blood flow reaching alveoli.

• Ventilation: air reaching alveoli.

• Local autoregulation matches V̇ to Q̇:
  • ↓Alveolar P{O2} → arteriolar constriction → blood diverted to better-ventilated alveoli.
  • ↑Alveolar P{CO2} → bronchiolar dilation → CO₂ elimination enhanced.

Transport of Respiratory Gases

• Oxygen
  • 1.5–2 % dissolved in plasma.
  • 98–98.5 % bound to hemoglobin (Hb).
  • Each Hb carries 4 \mathrm{O2} → 1 RBC (~250 × 10⁶ Hb) binds ≈1 billion \mathrm{O2}.

• Carbon Dioxide
  • 7–10 % dissolved.
  • 20 % bound to Hb (carbamino-Hb).
  • 70 % as bicarbonate ions in plasma.

Oxygen–Hemoglobin Chemistry

• Reversible reaction: \mathrm{HHb + O2 \leftrightarrow HbO2 + H^+}.

• Affinity changes with ligation:
  • Binding of first \mathrm{O_2} raises affinity (co-operativity).
  • Release lowers affinity.

• Saturation states:
  • 100 % → all 4 heme sites filled.
  • Partial (1–3 sites) common in venous blood.

Oxygen-Hemoglobin Dissociation Curve

• Sigmoidal (S-shape).

• Arterial blood (rest): P{O2}=100\,\text{mmHg}, Hb ~98 % saturated, content 20 vol %.

• Venous blood: P{O2}=40\,\text{mmHg}, Hb ~75 % saturated → 5 vol % “venous reserve”.

• Plateau (upper flat) provides a safety margin (altitude, lung disease).

• Steep portion facilitates unloading in tissues; small P{O2} drop → large O₂ release.

Factors Shifting the Curve

• Right shift (↓ affinity / ↑ unloading):
  • ↑Temperature.
  • ↑H^+ (↓pH).
  • ↑P{CO2}.
  • ↑BPG (2,3-bisphosphoglycerate).
  • Bohr effect: \uparrow P{CO2} or \downarrow pH weakens Hb-O₂ bond.

• Left shift (↑ affinity / ↓ unloading):
  • Opposite changes (↓Temp, ↓H^+, ↓P{CO2}, ↓BPG).

• Exercise & actively metabolizing tissues: heat, acid, & CO₂ production ↑ → right shift → enhanced O₂ delivery.

Five Major Modulators of O₂-Hb Binding

1. P{O2}. 2. Temperature. 3. pH (H^+). 4. P{CO2}. 5. BPG concentration.

Hypoxia Types

• Anemic: low Hb or RBC count.

• Ischemic: impaired blood flow.

• Histotoxic: cells cannot use O₂ (e.g., cyanide).

• Hypoxemic: ventilation deficit/pulmonary disease.

• CO poisoning: Hb affinity for CO >200× O₂; treated by hyperbaric O₂.

Carbon Dioxide Transport & Exchange

1. Dissolved 7{-}10\%.

2. Carbamino-Hb 20\%
   • Reaction: \mathrm{CO2 + Hb\text{-}NH2 \leftrightarrow Hb\text{-}NHCOO^- + H^+}.

3. Bicarbonate (major)\;70\%
   • Carbonic anhydrase (RBC) catalyzes:
   \mathrm{CO2 + H2O \leftrightarrow H2CO3 \leftrightarrow H^+ + HCO_3^-}.

Chloride Shift

• Systemic capillaries: HCO_3^- exits RBC → Cl^- enters to maintain electroneutrality.

• Pulmonary capillaries: reverse shift; HCO3^- re-enters, combines with H^+ → H2CO3 → CO2 + H2O → CO2 diffuses into alveoli.

Haldane Effect

• Lower P{O2} / reduced Hb → ↑CO₂ carriage.

• Facilitates CO₂ pick-up in tissues & release in lungs.

• Synergistic with Bohr effect (CO₂ promotes O₂ unloading, O₂ unloading promotes CO₂ loading).

Acid–Base Homeostasis

• Carbonic Acid–Bicarbonate Buffer:
  • Acts instantly: excess H^+ + HCO3^- → H2CO3; deficit H^+ → H2CO3 \rightarrow H^+ + HCO3^-.

• Respiratory contribution:
  • Hypoventilation → ↑P{CO2} → ↑H^+ (↓pH).
  • Hyperventilation → ↓P{CO2} → ↓H^+ (↑pH).
  • Can partially compensate metabolic acidosis/alkalosis.

CO₂ and O₂ Competition for Heme

• No direct competition: CO₂ binds globin (carbamino), not heme Fe; O₂ binds heme Fe.

• Exception: CO (carbon monoxide) competes for heme Fe and displaces O₂.

Practice / DIY Summary

1. State Dalton’s & Henry’s laws and their respiratory relevance.

2. List & explain 5 factors affecting O₂-Hb binding/unbinding.

3. Outline three CO₂ transport forms and associated reactions.