📖 Notes
LESSON 08 OF 16
Respiratory System — Gas Exchange
70 Square Metres of Exchange Surface
The alveoli pack a tennis court's worth of surface area into your chest. Understanding why that surface area matters — and what breaks it — is core MDCAT respiratory physiology.
Pathway
Nose to Alveoli
1
Nose/Mouth → Pharynx → Larynx
Air is warmed, filtered, and moistened on the way in.
2
Trachea → Bronchi → Bronchioles
The airway branches repeatedly, like an upside-down tree.
3
Alveoli — the exchange site
70 m² of surface area (a tennis court), wet and one cell thick for fast diffusion, surrounded by capillaries.
70m²
Alveolar surface area
500mL
Tidal volume (normal breath)
4800mL
Vital capacity (max in/out)
1200mL
Residual volume (always remains)
Gas Exchange & Mechanics
- O2: moves from alveoli (high pO2) into blood (low pO2)
- CO2: moves from blood (high pCO2) into alveoli
- Inspiration: diaphragm contracts and flattens, ribs expand, pressure drops — active process
- Expiration: passive — the diaphragm simply relaxes
💡
Why surface area matters
Diseases like emphysema destroy alveolar walls, reducing the 70m² surface area — this single change explains why gas exchange efficiency drops even though the lungs are still "there."