Pulmonary Compliance and Elasticity Overview

Overview

This lecture explains pulmonary compliance and elasticity, how they are defined mathematically, and how various physiological and pathological factors alter them in the lungs and chest wall.

Compliance and Elasticity: Definitions and Relationships

Compliance (C)
- Defined as change in volume over change in pressure:
  - C = ΔV / ΔP
- Describes stretchability or distensibility of lungs or chest wall.
- High compliance = structure stretches easily with little pressure.
Elasticity (E)
- Defined as change in pressure over change in volume:
  - E = ΔP / ΔV
- Describes resistance to stretch and tendency to recoil to smallest size.
- High elasticity = strong recoil, resists being stretched.
Directions of proportionality
- For compliance:
  - ΔV ∝ C (if volume change increases, compliance increases)
  - ΔP ∝ 1/C (if pressure needed increases, compliance decreases)
- For elasticity:
  - ΔP ∝ E (if elasticity increases, more pressure change is generated)
  - ΔV ∝ 1/E (if volume change increases, elasticity decreases)
Relevant pressures
- Lungs: transpulmonary pressure (TP) is the ΔP term for lung compliance.
- Chest wall: transthoracic pressure (TTP) is the ΔP term for chest wall compliance.

Normal Factors Affecting Compliance

Three main factors normally determine lung compliance:

Elasticity of the lungs
Surface tension in the alveoli
Elasticity (and mechanics) of the chest wall

Additional important influences:

Neuromuscular function of respiratory muscles
Airway obstruction by mucus
Pleural space conditions (e.g., pneumothorax, hemothorax)

Summary Table: Effects on Compliance and Elasticity

Factor / Condition	Primary Structure Affected	Effect on Compliance	Effect on Elasticity or Recoil	Main Functional Consequence
Normal lung tissue	Lungs	Normal (highish)	Normal (enough recoil)	Easy inflation and passive exhalation
Pulmonary fibrosis	Lung parenchyma	Decreases	Increases	Restrictive lung disease, hard to inflate lungs
Emphysema	Lung parenchyma	Increases	Decreases	Obstructive disease, hard to exhale air out
Ankylosing spondylitis	Chest wall and spine	Decreases	Functionally “stiff” chest wall	Difficult inspiration, restrictive pattern
Kyphosis	Chest wall and spine	Decreases	Functionally “stiff” chest wall	Difficult inspiration, restrictive pattern
Scoliosis	Chest wall and spine	Decreases	Functionally “stiff” chest wall	Difficult inspiration, restrictive pattern
Calcified rib cartilage (aging)	Chest wall	Decreases	Functionally “stiff” chest wall	Reduced chest expansion with age
High surface tension (IRDS)	Alveoli	Decreases	Strong collapse tendency	Very high work of breathing, alveolar collapse
Normal surfactant	Alveoli	Increases	Decreases collapse tendency	Easier inflation, lower work of breathing
Diaphragm paralysis	Respiratory muscles	Decreases	N/A mechanical deficit	Reduced ability to expand lungs
External intercostal paralysis	Respiratory muscles	Decreases	N/A mechanical deficit	Loss of bucket and pump handle movements
ALS damaging motor neurons	Motor neurons to muscles	Decreases	N/A mechanical deficit	Progressive respiratory failure
Mucus plugging (e.g., chronic bronchitis, cystic fibrosis)	Airways	Decreases	N/A reduced ventilation	Underventilated alveoli, poor expansion
Pneumothorax / hemothorax → atelectasis	Pleural space and lung	Decreases	Lung collapses inward	Markedly reduced lung volume on affected side

Elasticity of the Lungs

Normal situation
- Lungs are very compliant but also have just the right amount of elasticity.
- Elastic fibers allow easy expansion but provide inward recoil for passive exhalation.
- There is a balanced interplay between lung compliance and elasticity.
Pulmonary fibrosis
- Fibrous (scar) tissue replaces normal, distensible lung tissue.
- Fibrous tissue is not distensible and resists stretch.
- Decreased lung volume change for a given pressure.
Emphysema
- Classified as a chronic obstructive pulmonary disease.
- Destruction of elastic tissue in alveolar walls by elastase from neutrophils.
- Loss of alveolar surface area and elastic recoil.
- Lungs become “floppy,” overly compliant, but cannot recoil effectively.

Pulmonary Fibrosis vs Emphysema

Pulmonary fibrosis
- More fibrous tissue, more resistance to stretch.
- Compliance decreases.
- Elasticity and recoil increase.
- Lungs resist inflation, causing low forced vital capacity (restrictive pattern).
Emphysema
- Breakdown of elastic fibers in alveoli.
- Compliance increases (lungs very easy to inflate).
- Elasticity decreases (poor recoil).
- Difficult to exhale passively, causing low forced expiratory volume.

Elasticity and Compliance of the Chest Wall

Normal chest wall mechanics
- Chest wall has its own compliance and elasticity.
- Lungs have inward recoil; chest wall tends to recoil outward.
- Dynamic interplay keeps intrapleural pressure negative and volumes in balance.
Conditions that decrease chest wall compliance
- Ankylosing spondylitis
  - Inflammatory arthritis causing stiff, hunched posture.
  - Chest cavity bent, reduced expansion capacity.
- Kyphosis
  - Hunched-over spinal curvature, similar restrictive effect.
- Scoliosis
  - Abnormal S-shaped spine, distorts thoracic cage mechanics.
- Calcification of rib cartilage with aging
  - Ribs become less flexible, limiting chest expansion.
Effects on breathing
- In these conditions, chest wall cannot expand fully.
- Decreased chest wall compliance.
- Inspiration becomes difficult; tidal volumes reduced.
- Often contributes to restrictive lung physiology.
Hypothetical high chest wall compliance
- Very flexible chest wall would expand easily.
- Increased chest wall compliance.
- Could allow more air intake during inspiration.

Surface Tension and Surfactant

Surface tension in alveoli
- Thin water layer lines alveoli; air–water interface creates surface tension.
- Surface tension tends to shrink and collapse alveoli to smallest possible size.
- If many alveoli shrink, overall lung volume decreases and inflation becomes difficult.
Increased surface tension: Infant Respiratory Distress Syndrome (IRDS)
- Infants do not produce enough surfactant (a protein–lipid complex).
- Surface tension remains high, strongly collapsing alveoli.
- Enormous effort required to inflate alveoli with each breath.
- Compliance decreases, and work of breathing increases greatly.
- Often requires mechanical ventilation.
Role of surfactant (normal situation)
- Produced by type II alveolar cells.
- Decreases surface tension at the air–water interface in alveoli.
- Reduces alveolar tendency to collapse.
- Allows alveoli, and thus lungs, to expand more easily.
Effect of decreased surface tension via surfactant
- Alveoli more likely to remain open and expand.
- Lung compliance increases.
- Work of breathing decreases.

Neuromuscular Effects on Compliance

Importance of respiratory muscles
- Diaphragm and external intercostal muscles generate the mechanical forces for inspiration.
- Their ability to contract determines how well the lungs can expand.
Diaphragm injury or paralysis
- Damaged diaphragm cannot contract.
- Inspiration is severely impaired.
- Lungs cannot expand normally.
- Lung compliance effectively decreases due to mechanical limitation.
External intercostal paralysis
- External intercostals normally elevate ribs (bucket handle) and sternum (pump handle).
- Paralysis stops these movements, limiting chest expansion.
- Decreases ventilatory capacity and apparent compliance.
ALS (Amyotrophic Lateral Sclerosis)
- Degenerative disease affecting motor neurons in the ventral gray horn of the spinal cord.
- Neurons from the VRG (ventral respiratory group) project to these motor neurons.
- ALS destroys these neurons so action potentials cannot reach diaphragm or intercostal muscles.
- Respiratory muscles fail to contract, reducing lung expansion and compliance.

Airway Mucus and Ventilation

Mucus accumulation in airways
- Occurs in conditions such as chronic bronchitis and cystic fibrosis.
- Excess mucus fills bronchi and bronchioles and obstructs airflow.
Consequences for compliance
- Air cannot reach distal alveoli effectively.
- Alveoli become underventilated and do not expand well.
- Overall lung expansion is reduced.
- Lung compliance decreases.

Pleural Pressure, Pneumothorax, Hemothorax, and Atelectasis

Normal pressures
- Intrapleural pressure (P_ip) ≈ −4 mmHg relative to atmosphere.
  - At atmospheric pressure of 760 mmHg, P_ip ≈ 756 mmHg.
- Intrapulmonary (alveolar) pressure (P_alv) ≈ 0 mmHg (≈ 760 mmHg absolute) at rest.
- Pressure difference between P_alv and P_ip keeps lungs expanded.
Penetrating chest injury: development of pneumothorax
- Puncture through chest wall and parietal pleura opens pleural cavity to atmosphere.
- Atmospheric pressure = 760 mmHg, pleural cavity initially ≈ 756 mmHg.
- Air flows from high pressure (atm) to lower pressure (pleural space).
- Air accumulates in pleural cavity: pneumothorax.
Hemothorax
- Damage or infection causes blood to leak from pulmonary capillaries into pleural space.
- Blood accumulates in pleural cavity: hemothorax.
General pleural effusions (air, blood, other fluids)
- Any substance accumulating in pleural cavity raises intrapleural pressure.
- P_ip increases until it equals atmospheric pressure (760 mmHg, or 0 mmHg relative).
- If P_ip equals or exceeds P_alv, it compresses lungs.
Atelectasis (lung collapse)
- Elevated intrapleural pressure pushes on lung, causing it to collapse.
- Collapsed lung is small with minimal volume.
- Recoil tendency (elasticity) effectively dominates as lung shrinks.
- Lung volume decreases markedly; compliance decreases with lower volume.

Key Terms & Definitions

Compliance (C): Measure of how easily the lungs or chest wall stretch; C = ΔV / ΔP.
Elasticity (E): Measure of recoil or resistance to stretch; E = ΔP / ΔV.
Transpulmonary pressure (TP): Pressure difference across lung wall; determines lung expansion.
Transthoracic pressure (TTP): Pressure difference across chest wall; determines chest wall expansion.
Pulmonary fibrosis: Disease with excessive fibrous scar tissue in lungs, reducing distensibility.
Emphysema: Chronic obstructive lung disease with destruction of alveolar walls and elastic fibers.
Ankylosing spondylitis: Inflammatory arthritis causing spinal fusion and hunched posture.
Kyphosis: Excessive forward curvature of thoracic spine (hunched back).
Scoliosis: Lateral curvature of spine with possible S-shaped deformity.
Surface tension: Force at fluid–air interface in alveoli, tending to shrink and collapse them.
Surfactant: Protein–lipid complex from type II alveolar cells that lowers alveolar surface tension.
Infant Respiratory Distress Syndrome (IRDS): Condition in infants with deficient surfactant and high surface tension.
Diaphragm: Primary inspiratory muscle separating thoracic and abdominal cavities.
External intercostal muscles: Muscles between ribs elevating ribs and sternum during inspiration.
VRG (Ventral Respiratory Group): Neuronal group in brainstem driving respiratory motor neurons.
ALS (Amyotrophic Lateral Sclerosis): Neurodegenerative disease destroying motor neurons in ventral gray horn.
Pneumothorax: Presence of air in pleural cavity.
Hemothorax: Presence of blood in pleural cavity.
Pleural effusion: General term for accumulation of fluid, air, or blood in pleural space.
Intrapleural pressure (P_ip): Pressure within pleural cavity, normally negative relative to atmosphere.
Intrapulmonary (alveolar) pressure (P_alv): Pressure inside lung alveoli.
Atelectasis: Collapse of part or all of a lung, reducing its volume.

Action Items / Next Steps

Memorize definitions and formulas for compliance and elasticity and their proportional relationships with pressure and volume.
Be able to predict how specific diseases (fibrosis, emphysema, IRDS, chest wall deformities, neuromuscular disorders) change compliance and elasticity.
Practice relating intrapleural and intrapulmonary pressures to lung expansion, pneumothorax, and atelectasis scenarios.
Review roles of diaphragm, external intercostals, surfactant, and chest wall structure in normal breathing mechanics.