Defense Systems: How Lungs Fight Back
Learning Objectives
Students will be able to:
- Describe the respiratory system's multiple layers of defense
- Explain how mucus and cilia work together (mucociliary escalator)
- Identify the role of macrophages in the alveoli
- Predict which defenses work against which particle sizes
The Problem
You breathe in 10,000+ liters of air every day. That air contains:
- Dust and dirt particles
- Pollen and mold spores
- Bacteria and viruses
- Smoke and pollution particles
- Allergens from pets, dust mites, etc.
How does your body keep all this out of your delicate lungs?
Defense Layer 1: The Nose
Physical Filters
- Nose hairs (vibrissae) — Trap large particles (>10 μm)
- Narrow passages — Force air through twists and turns
- Mucus lining — Sticky coating catches particles
Conditioning
- Warming — Blood vessels warm cold air
- Humidifying — Adds moisture
- This is why nose breathing is better than mouth breathing!
Defense Layer 2: The Mucociliary Escalator
The airways are lined with a remarkable two-part cleaning system:
Mucus (The Trap)
- Sticky gel layer covering airways
- Traps particles that land on it
- You produce 1-2 liters per day!
- Contains antibodies and enzymes
Cilia (The Movers)
- Tiny hair-like structures
- Beat 10-20 times per second
- Sweep mucus upward like an escalator
- Move mucus at ~1 cm/minute
The escalator moves particles up to the throat where they're swallowed (into stomach acid) or coughed out.
How the Escalator Works
THROAT (swallow or cough)
↑
========================================
~~~~~~~~ MUCUS LAYER (sticky) ~~~~~~~~~ ← Particles trapped here
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||||||||||||||||||||||||||||||||||||||||| ← CILIA beating upward
|||||||||||||||||||||||||||||||||||||||||
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AIRWAY WALL
Think of it like a conveyor belt at an airport, but going UP from your lungs to your throat!
Defense Layer 3: Coughing and Sneezing
| Reflex | Triggered By | Air Speed |
|---|---|---|
| Cough | Irritation in trachea/bronchi | Up to 500 mph! |
| Sneeze | Irritation in nose | Up to 100 mph |
These explosive reflexes eject particles, mucus, and irritants that the escalator can't handle fast enough.
Defense Layer 4: Alveolar Macrophages
The Last Line of Defense
Particles that make it all the way to the alveoli encounter macrophages:
- Macrophage = "Big eater" in Greek
- Immune cells that patrol the alveoli
- Engulf and digest particles, bacteria, and debris
- One of the body's most important defenses
Problem: Macrophages can be overwhelmed by too many particles, or damaged by toxic particles. They can't effectively eat particles smaller than about 0.5 μm.
Which Defense Works Against Which Particles?
| Particle Size | Where Deposited | Defense Mechanism | Effectiveness |
|---|---|---|---|
| >10 μm | Nose/mouth | Nose hairs, sneezing | Very effective |
| 5-10 μm | Trachea/bronchi | Mucociliary escalator, coughing | Effective |
| 1-5 μm | Bronchioles | Escalator (slower here) | Moderate |
| 0.5-1 μm | Alveoli | Macrophages | Moderate |
| <0.5 μm | Alveoli/bloodstream | Limited! | Poor |
What Damages Our Defenses?
Harmful Factors
- Smoking — Paralyzes cilia, increases mucus
- Air pollution — Overwhelms defenses
- Cold, dry air — Thickens mucus
- Dehydration — Mucus becomes too sticky
- Some medications — Can dry out airways
Helpful Factors
- Clean air — Less work for defenses
- Good hydration — Keeps mucus flowing
- Nose breathing — Uses full defense system
- Humid air — Easier ciliary action
- Not smoking — Protects cilia
Key Takeaway
The respiratory system has multiple layers of defense: nose hairs and mucus for large particles, the mucociliary escalator for medium particles, coughing for irritants, and macrophages for what reaches the alveoli. These defenses work best against larger particles and struggle with PM2.5 and smaller. The best strategy is to keep harmful particles out of the air in the first place!