🫁 Dynamic Lung Mechanics

How is airflow generated in the lungs?

• Flow = ΔP / Resistance (Fick equation)

• ΔP = alveolar pressure (PA) – barometric pressure (PB)

• Airflow ↑ if pressure gradient ↑ OR airway resistance ↓

What is airway resistance (RAW)?

• Defined as ΔP / Flow

• Falls with bronchodilation (sympathetic outflow, exercise) → higher flow at same ΔP

• Dominated by radius (r⁴) per Poiseuille’s law: R=8ηlπr4R = \frac{8 \eta l}{\pi r^4}

How does total cross-sectional area (CSA) change across the lung?

• Individual airway diameter ↓ deeper in lung

• Total CSA ↑ exponentially (branches: 1, 2, 4, 16, 256…)

• Result: resistance falls precipitously after initial generations

Formula for resistance in series vs parallel?

• Series: R_total = R1 + R2 + R3 → high resistance, low flow (not physiologic)

• Parallel: 1/R_total = 1/R1 + 1/R2 + … → resistance falls dramatically

• Lung behaves like parallel resistances → typical RAW ≈ 1 cm H₂O/L/s

What factors contribute to airway resistance?

• Radius (r⁴ effect)

• Length of airway

• Gas viscosity

• Elastic tendency to collapse (prevented by interdependence + pleural pressure)

What is interdependence?

• Airways support each other via connective tissue links

• Reduced in COPD → collapse risk ↑

How does lung volume affect resistance?

• Resistance ↑ as lung volume ↓ (airway radius shrinks)

• COPD patients: higher resistance at all volumes → they breathe at higher lung volumes to keep airways open

What are the types of airflow in the tracheobronchial tree?

• Laminar: ordered, ΔP alone determines flow

• Transitional: laminar in straight tubes, turbulence at branch points

• Turbulent: disordered, flow ∝ √ΔP → requires greater driving pressure

What is Reynolds number (Re)?

• Predicts turbulence: Re > 2000 → turbulence ↑

• Depends on radius, velocity, density, viscosity

• Trachea: turbulent flow

• Small airways: laminar (low velocity, diffusion dominates)

• COPD: reduced diameter → velocity ↑ → Re ↑ → turbulence ↑ → resistance ↑

Define intrapleural pressure (PIP).

• Pressure in pleural space between lung & chest wall

• Negative, keeps lung adhered to rib cage

Define transpulmonary pressure (PTP).

• PTP = PA – PIP

• Static parameter, determines lung volume

• Fights elastic recoil tendency to collapse lung

Define alveolar pressure (PA).

• Pressure inside alveoli

• Static: maintains lung volume

• Dynamic: changes during cycle → drives airflow

Difference between airway resistance vs conductance?

• Resistance: mmHg/L/sec → pressure development

• Conductance: L/sec/mmHg → airflow

• High lung volume → resistance ↓ → conductance ↑

What is dynamic compliance?

• Pressure-volume slope during tidal breathing

• Exercise: bronchodilation ↑ compliance → higher lung volumes, faster airflow

Impact of altered resistance/compliance?

• Normal: alveoli fill equally

• ↑ Resistance: slow filling, flow diverted to healthy alveoli → limits frequency

• ↓ Compliance: alveoli fill less, need ↑ ΔP → ↑ work of breathing

What is dynamic airway compression?

• Passive exhalation: PA > PIP → no collapse

• Forced exhalation: PIP ↑, PA bleeds off → collapse point when PIP > PA

• Occurs in small bronchioles (no cartilage)

• Airflow continues only due to pressure difference collapse point → mouth

• Slows expiration → hallmark of COPD

Components of work of breathing?

• Elastic work: overcome chest wall recoil + lung tissue elasticity

• Resistive work: overcome airway resistance + tissue viscosity

• Breathing rate chosen to minimize total work (12–20 breaths/min)

What pressures change across the respiratory cycle?

• PA (alveolar): dynamic, drives airflow

• PIP (intrapleural): negative, keeps lung adhered

• PTP (transpulmonary): static, determines lung volume