Gas Exchange Physiology: How Oxygen Enters & Carbon Dioxide Leaves the Body

Gas exchange is the vital process by which oxygen (O₂) enters the bloodstream and carbon dioxide (CO₂) is removed. It occurs continuously in the alveoli of the lungs.

Step-by-Step Process

1. Ventilation (Inhalation): Air enters via nose/mouth → trachea → bronchi → bronchioles → alveolar sacs.
2. Alveolar Structure: ~480 million alveoli; walls one cell thick (~0.5 µm); surrounded by dense capillary network. Total surface area ~70 m².
3. Diffusion (O₂): Partial pressure gradient drives O₂ from alveolar air (PₐO₂ ~100 mmHg) into capillary blood (PvO₂ ~40 mmHg). O₂ binds to hemoglobin (Hb) in RBCs.
4. Transport: Oxyhemoglobin travels via pulmonary veins → left heart → systemic arteries → tissue capillaries.
5. Tissue Exchange: O₂ dissociates from Hb (facilitated by low pO₂, high pCO₂, low pH, heat — Bohr effect) and diffuses into cells for mitochondrial respiration.
6. CO₂ Production: Cellular metabolism generates CO₂; diffuses into capillaries.
7. CO₂ Transport: Three forms: dissolved (10%), bicarbonate (HCO₃⁻, 70% via carbonic anhydrase), carbamino-Hb (20%).
8. Exhalation: CO₂ diffuses from pulmonary capillaries (PvCO₂ ~45 mmHg) into alveoli (PₐCO₂ ~40 mmHg) → exhaled.

Key Principles

• Fick's Law: Diffusion rate ∝ (Area × ΔP) / Thickness. Emphysema ↓ area; fibrosis ↑ thickness; both impair exchange.
• Ventilation-Perfusion (V/Q) Matching: Optimal when alveolar airflow matches capillary blood flow. Mismatch causes hypoxemia.
• Oxyhemoglobin Dissociation Curve: Sigmoidal shape enables loading in lungs (high pO₂) and unloading in tissues (low pO₂).

Clinical Relevance

Impaired gas exchange underlies respiratory failure (hypoxemic, hypercapnic), guides ventilator management, and explains altitude/hypoxia physiology.

This elegant, passive process sustains every aerobic cell in the body.