Gas Exchange: The Magic of Alveoli
Learning Objectives
Students will be able to:
- Explain how diffusion drives gas exchange in the lungs
- Describe what happens at the alveolar-capillary interface
- Compare the composition of inhaled and exhaled air
- Connect gas exchange to cellular respiration
The Big Picture
Every cell in your body needs oxygen and produces carbon dioxide.
The respiratory system's job is to get O2 from the air into your blood, and CO2 from your blood into the air. This exchange happens in the alveoli.
Diffusion: The Driving Force
Diffusion = Movement of molecules from high concentration to low concentration
HIGH in alveoli (just inhaled) → LOW in blood (cells used it up)
↓ Diffuses INTO blood
HIGH in blood (cells produced it) → LOW in alveoli
↓ Diffuses OUT of blood
No energy required! Diffusion is passive—it happens automatically because molecules naturally move from crowded areas to less crowded areas.
Inhaled vs. Exhaled Air
| Gas | Inhaled Air | Exhaled Air | Change |
|---|---|---|---|
| Nitrogen (N2) | 78% | 78% | No change (we don't use it) |
| Oxygen (O2) | 21% | 16% | ↓ 5% absorbed |
| Carbon Dioxide (CO2) | 0.04% | 4% | ↑ 100× increase! |
| Water vapor | Variable | ~6% | ↑ Saturated from lungs |
This is why exhaled air feels warm and moist!
The Alveolar-Capillary Interface
Gas exchange happens where alveoli meet blood capillaries:
| Alveolar wall | 1 cell thick (~0.2 μm) |
| Capillary wall | 1 cell thick (~0.2 μm) |
| Total barrier | Only ~0.5 μm! |
| Comparison | That's 1/100th the width of a human hair |
Why so thin? Gases diffuse faster through thinner barriers. This ultra-thin membrane allows rapid exchange.
The Exchange Process
ALVEOLUS (air sac)
============================
| |
| O2 O2 O2 O2 | ← Fresh oxygen from breath
| ↓ ↓ ↓ ↓ |
============================ ← Thin membrane (0.5 μm)
============================
| ↑ ↑ ↑ ↑ |
| CO2 CO2 CO2 CO2 | → CO2 exits to be exhaled
| BLOOD FLOWING → |
============================
CAPILLARY
Blood spends only about 0.75 seconds in contact with each alveolus—but that's enough time for complete gas exchange!
Why This Matters for Air Quality
The Problem
- The thin membrane that helps gas exchange...
- ...also lets tiny particles and chemicals through
- PM2.5 can reach alveoli
- Ultrafine particles can cross into blood
- This is how air pollution affects the whole body
The Protection
- Keep particles out in the first place
- Filtration and ventilation help
- Body has defense mechanisms (next lesson)
- Clean air = healthy gas exchange
Demonstration: Limewater CO2 Test
Materials:
- Clear limewater solution (calcium hydroxide)
- Two test tubes or clear containers
- Straw
Procedure:
- Fill both containers with equal amounts of limewater
- In container 1: Blow exhaled air through the straw into the limewater
- Container 2: Keep as control (just room air)
- Observe what happens to each container
Expected result:
The limewater you breathed into turns milky/cloudy. The CO2 in your exhaled breath reacts with calcium hydroxide to form calcium carbonate (chalk). This proves exhaled air contains much more CO2 than room air!
Key Takeaway
Gas exchange in the alveoli happens by diffusion—oxygen moves from air to blood while carbon dioxide moves from blood to air. This exchange is possible because of the incredibly thin barrier (just 0.5 μm) between alveoli and blood capillaries. The same thin barrier that enables life-sustaining gas exchange also means particles and pollutants in air can potentially reach our blood.