Study Notes: Lung Compliance and Pulmonary Mechanics

Lung Compliance — how easily the lungs expand when pressure is applied.

1. Definition of Compliance

Compliance = ΔV / ΔP
→ Change in Volume / Change in Pressure

• Describes the distensibility (expandability) of the lungs.

• High compliance = lungs expand easily.

• Low compliance = lungs are stiff and hard to expand.

Transmural (Transpulmonary) Pressure = Alveolar pressure − Intrapleural pressure
→ This is the driving pressure that expands the lungs.

2. Relationship Between Compliance, Elasticity, and Surface Tension

Concept	Function	Direction
Compliance	Allows lungs to expand	Expand
Elasticity / Recoil	Makes lungs collapse	Collapse
Surface Tension	Promotes alveolar collapse	Collapse

Opposites:

• Compliance ↔ Elasticity

• Compliance ↔ Surface tension

3. Real-Life Analogy: Socks Example

Analogy	Explanation	Disease Type
Normal Socks	Expand normally, recoil normally	Normal lung
Old socks (loose rubber band)	Easy to expand (↑ compliance), don’t recoil (↓ elasticity)	Emphysema (Obstructive)
Shrunk socks (tight)	Hard to expand (↓ compliance), strong recoil	Pulmonary Fibrosis (Restrictive)

Summary:

• Emphysema → ↑ Compliance, ↓ Recoil

• Fibrosis → ↓ Compliance, ↑ Recoil

4. Compliance vs. Recoil

Feature	Compliance	Recoil
Meaning	Lung expansion ability	Lung collapsing force
Increased in	Emphysema	Fibrosis
Decreased in	Fibrosis	Emphysema
Affected by	Surfactant, elastase	Surface tension, elastin

5. Obstructive vs Restrictive Lung Disease

Type	Problem	Example	Compliance	Key Symptom
Obstructive	Trouble getting air out	Emphysema, COPD, Asthma	↑	Air trapping
Restrictive	Trouble getting air in	Pulmonary Fibrosis	↓	Small lung volumes

Mnemonics:

• Obstructed = Out problem

• Restricted = In problem

6. Surfactant and Surface Tension

• Surface tension: Force trying to collapse alveoli.

• Surfactant (produced by Type II pneumocytes): Reduces surface tension → ↑ Compliance.

• No surfactant (e.g. neonatal RDS): ↑ Surface tension → ↓ Compliance → Lung collapse.

7. Saline-Filled vs Air-Filled Lungs

Type	Has Surface Tension?	Recoil	Compliance
Air-filled lung	Yes	High	Lower
Saline-filled lung	No	Low	Higher

Reason:
No air-fluid interface → No surface tension → Easier expansion (↑ compliance).
Therefore, saline-filled lungs have greater compliance.

8. Expiration vs Inspiration Compliance

• Expiration curve has greater compliance than inspiration.

Why?

• During inspiration, surface tension must be overcome → requires more pressure.

• During expiration, alveoli are already open → less pressure needed.

Proof (3 methods):

1. At same pressure, expiratory volume > inspiratory volume → ↑ compliance.

2. At same volume, expiration requires less pressure.

3. Graph slope (ΔV/ΔP) steeper in expiration → ↑ compliance.

9. Alpha-1 Antitrypsin Deficiency

• Mechanism:
  ↓ Alpha-1 antitrypsin → ↑ Elastase activity → destroys elastin → ↓ Recoil → ↑ Compliance

• Effect:
  Easy to inhale (air in), hard to exhale (air trapped) → Obstructive pattern

10. Pressure–Volume Loops

• Plot Volume (Y-axis) vs Pressure (X-axis)

• Used to compare:

  • Normal lung

  • Emphysematous lung (↑ slope = ↑ compliance)

  • Fibrotic lung (↓ slope = ↓ compliance)

Slope = Compliance = ΔV / ΔP
→ Steeper slope = higher compliance.

11. Chest Wall vs Lung Compliance

• Chest wall naturally expands outward.

• Lungs naturally recoil inward.

• Together, they balance to create a negative intrapleural pressure.

• The combined compliance (lung + chest wall) is less than either alone.

12. Quick Summary Table

Parameter	Emphysema (Obstructive)	Fibrosis (Restrictive)
Compliance	↑ Increased	↓ Decreased
Recoil	↓ Decreased	↑ Increased
Air In (Inspiration)	Easy	Difficult
Air Out (Expiration)	Difficult	Easy
Elastic Tissue	Destroyed (↓ elastin)	Excess collagen
Example Analogy	Old socks	Shrunk socks

13. Key Equations and Takeaways

• Compliance = ΔV / ΔP

• Inverse Relationship: Compliance ↔ Recoil

• Surfactant ↓ surface tension → ↑ Compliance

• Obstructive = outflow problem (↑ compliance)

• Restrictive = inflow problem (↓ compliance)

14. Case Example

Three subjects (A, B, C):

1. Normal lung → moderate compliance

2. Emphysema → high compliance

3. Amiodarone/Bleomycin (fibrosis) → low compliance

15. Key Quote

“The negative intrapleural pressure is due to the dynamic harmonious antagonism between the chest wall (expands) and the lung (recoils).”

Core Memory Aids

• Right = Release (O₂ curve mnemonic from previous lesson)

• Compliance = Expand

• Elasticity = Recoil

• Obstructive → Out problem

• Restrictive → In problem