Every breath you take is an act of gas trade. You inhale oxygen to fuel your cells and exhale carbon dioxide — the waste product of metabolism. This exchange happens in your lungs and your tissues, all governed by pressure gradients and diffusion.
🫁 Overview: The Pathway of Gases
| Process | Location | Description |
|---|---|---|
| Ventilation | Lungs ↔ Atmosphere | Movement of air in/out of alveoli |
| External respiration | Alveoli ↔ Pulmonary capillaries | O₂ in, CO₂ out |
| Transport | Blood | Gases carried to/from tissues |
| Internal respiration | Systemic capillaries ↔ Cells | O₂ diffuses into cells, CO₂ out |
| Cellular respiration | Mitochondria | Use of O₂ for ATP, production of CO₂ |
⚙️ 1️⃣ Gas Exchange in the Lungs (External Respiration)
Gas exchange occurs in the alveoli, across the respiratory membrane, which is only 0.5 μm thick — designed for rapid diffusion.
Respiratory Membrane Components:
- Alveolar epithelium (Type I cells)
- Fused basement membrane
- Capillary endothelium
Thickness: <1 μm — the thinner it is, the faster diffusion occurs.
Partial Pressures — The Driving Force
Gases move from high → low partial pressure (Dalton’s law). The difference in partial pressures between alveoli and blood drives diffusion.
| Gas | Alveolar (mmHg) | Pulmonary Artery (mmHg) | Pulmonary Vein (mmHg) |
|---|---|---|---|
| O₂ | 104 | 40 | 100 |
| CO₂ | 40 | 45 | 40 |
🩸 2️⃣ Transport of Oxygen
Only a tiny fraction of oxygen travels freely in plasma — most is carried by hemoglobin (Hb) inside RBCs.
A. Oxygen in Blood
| Form | Percentage | Description |
|---|---|---|
| Bound to Hb | ~98.5% | Each Hb molecule binds up to 4 O₂ molecules |
| Dissolved in plasma | ~1.5% | Creates PO₂ that drives diffusion |
B. Oxygen–Hemoglobin Dissociation Curve
This famous S-shaped curve shows how Hb’s affinity for O₂ changes with PO₂.
| PO₂ (mmHg) | % Hb Saturation |
|---|---|
| 100 (lungs) | 97–100% |
| 40 (tissues) | ~75% |
| 20 (active muscles) | ~25% |
C. Factors Shifting the Curve
| Shift Direction | Cause | Effect on Hb Affinity | Physiological Example |
|---|---|---|---|
| Right shift | ↑ CO₂, ↑ temperature, ↓ pH, ↑ 2,3-BPG | ↓ Affinity → more O₂ released | Exercise, fever |
| Left shift | ↓ CO₂, ↓ temp, ↑ pH, ↓ 2,3-BPG | ↑ Affinity → less O₂ released | Fetal Hb, hypothermia |
🧠 Bohr Effect
↑ CO₂ or ↓ pH (acidic blood) → Hb releases more O₂. Enhances oxygen unloading in metabolically active tissues.
🌬️ 3️⃣ Transport of Carbon Dioxide
CO₂ is continuously produced by tissues and must be carried to the lungs for exhalation. It’s transported in three forms:
| Form | Percentage | Mechanism |
|---|---|---|
| As bicarbonate (HCO₃⁻) | ~70% | CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻ (via carbonic anhydrase) |
| As carbaminohemoglobin (Hb–CO₂) | ~23% | Binds to Hb’s globin (not heme) |
| Dissolved in plasma | ~7% | Directly soluble in plasma |
Haldane Effect
Deoxygenated Hb binds CO₂ more readily. So in tissues, where O₂ leaves Hb, CO₂ uptake is enhanced. In lungs, O₂ loading promotes CO₂ release.
🫀 5️⃣ Gas Exchange in the Tissues (Internal Respiration)
At tissue level: PO₂ in tissues ≈ 40 mmHg; PCO₂ in tissues ≈ 45 mmHg → O₂ diffuses from blood → tissues, and CO₂ diffuses from tissues → blood.
🧩 6️⃣ Factors Affecting Gas Exchange
| Factor | Effect if Altered | Clinical Example |
|---|---|---|
| Surface area | ↓ → ↓ diffusion | Emphysema |
| Membrane thickness | ↑ → ↓ diffusion | Pulmonary fibrosis, edema |
| Diffusion coefficient | ↓ → ↓ diffusion | CO₂ diffuses 20× faster than O₂ |
| Partial pressure gradient | ↓ → ↓ diffusion | High altitude |
| Ventilation–perfusion (V/Q) ratio | Mismatch → hypoxemia | Asthma, PE |
⚖️ 7️⃣ Ventilation–Perfusion (V/Q) Ratio
Normal: ≈ 0.8 (ventilation 4 L/min, perfusion 5 L/min). V/Q mismatch → major cause of hypoxemia.
| Condition | V/Q Change | Effect |
|---|---|---|
| Airway obstruction (shunt) | ↓ V/Q (approaches 0) | Blood passes unoxygenated |
| Pulmonary embolism (dead space) | ↑ V/Q (approaches ∞) | Ventilated but unperfused lung area |
💡 Clinical Applications
| Condition | Key Change | Effect |
|---|---|---|
| Anemia | ↓ Hb concentration | ↓ O₂ content, but normal PaO₂ |
| Carbon monoxide poisoning | Hb binds CO > O₂ | ↓ O₂ delivery, cherry-red skin |
| COPD / asthma | V/Q mismatch | Hypoxemia, hypercapnia |
| High altitude | ↓ atmospheric PO₂ | Hypoxemia → hyperventilation |
| Emphysema | ↓ surface area | Impaired O₂ diffusion |
🧠 Summary Table — Gas Transport
| Gas | Main Transport Form | Important Concept |
|---|---|---|
| O₂ | 98% as oxyhemoglobin | Bohr effect |
| CO₂ | 70% as bicarbonate | Haldane effect |
| N₂ | Insoluble → inert gas | Diving decompression risk |