Mechanics of Breathing

Muscles & Pressures - Pressure Players

• Inspiratory Muscles:

  • Principal: Diaphragm (main, flattens), External Intercostals (elevate ribs).
  • Accessory (forced): SCM, Scalenes, Pectoralis.

• Expiratory Muscles:

  • Quiet: Passive (elastic recoil).
  • Forced: Internal Intercostals, Abdominal muscles.

• Key Pressures (relative to P_atm = 0 cm H₂O):

  • P_alv (Intra-alveolar): Airflow driver. Inspiration -1, Expiration +1 cm H₂O.
  • P_ip (Intrapleural): Pressure in pleural cavity; crucial for lung inflation.

  ⭐ Intrapleural pressure is normally negative (approx. -5 cm H₂O at end-expiration, -8 cm H₂O at end-inspiration) due to opposing elastic recoils of lung and chest wall.

  • P_tp (Transpulmonary): $P_{tp} = P_{alv} - P_{ip}$. Alveolar distending pressure.

Lung Volumes & Capacities - Breath by Numbers

• Volumes:
  • Tidal Volume (TV): Air/quiet breath; ~500 mL.
  • Inspiratory Reserve (IRV): Max extra air inhaled; ~3000 mL.
  • Expiratory Reserve (ERV): Max extra air exhaled; ~1100 mL.
  • Residual Volume (RV): Air left after max exhale; ~1200 mL.
• Capacities: (Sums of volumes) 📌 Mnemonic: "Can I Find Vitality Today?" (IC, FRC, VC, TLC)
  • Inspiratory (IC): $TV + IRV$; ~3500 mL.
  • Functional Residual (FRC): $ERV + RV$; ~2300 mL.
  • Vital (VC): $IRV + TV + ERV$; ~4600 mL.
  • Total Lung (TLC): $VC + RV$; ~5800 mL.

⭐ Residual Volume (RV), Functional Residual Capacity (FRC), and Total Lung Capacity (TLC) cannot be measured by simple spirometry; require helium dilution or body plethysmography.

Lung Compliance & Surfactant - Stretch & Snap

• Lung Compliance ($C_L$): Lung/chest wall stretchability. $C_L = \Delta V / \Delta P$.
  • Normal (lungs): ~200 mL/cm H₂O.
  • ↑ $C_L$: Emphysema, aging (easier inflation).
  • ↓ $C_L$: Fibrosis, edema, ARDS, NRDS (stiffer lungs).
• Elastic Recoil: Lungs' tendency to deflate post-stretch; opposes compliance.
• Surface Tension: Alveolar air-liquid interface force causing collapse.
  • Laplace's Law: $P = 2T/r$. Smaller alveoli = ↑ collapse risk.
• Pulmonary Surfactant:
  • From Type II pneumocytes; main: Dipalmitoylphosphatidylcholine (DPPC).
  • Action: ↓ surface tension (esp. small alveoli), ↑ compliance, prevents atelectasis.
  • Deficiency: Neonatal RDS.

  ⭐ Surfactant (DPPC) reduces surface tension more effectively in smaller alveoli (greater concentration as surface area ↓), preventing collapse, per Laplace's Law ($P = 2T/r$).

Airway Resistance & Work - Flow & Force

• Airway Resistance (Raw): Opposition to airflow. $Raw = \frac{\Delta P}{\dot{V}}$ (Pressure gradient / Flow rate).
  • Poiseuille's Law: $R \propto \frac{\eta l}{r^4}$; radius ($r$) is dominant. $\eta$=viscosity, $l$=length.
  • $\downarrow$Radius (e.g., bronchoconstriction, mucus) $\rightarrow$ significantly $\uparrow$Raw.
  • $\uparrow$Lung Volume $\rightarrow$ $\downarrow$Raw (radial traction on airways).
  • Flow types: Laminar (smooth, low Raw), Turbulent (disordered, $\uparrow$Raw, e.g., trachea, high flow rates). Reynolds number ($Re$) predicts turbulence.
• Work of Breathing (WOB): Energy expended for ventilation. $W = P \times \Delta V$.
  • Components: Elastic work (to overcome lung/chest wall recoil) & Resistive work (to overcome Raw).
  • $\uparrow$Elastic work in restrictive diseases (e.g., fibrosis).
  • $\uparrow$Resistive work in obstructive diseases (e.g., asthma, COPD).
  • $O_2$ cost of breathing: Normally ~1-2% of total $O_2$ consumption; can $\uparrow$ significantly in respiratory disease.

⭐ The medium-sized bronchi (generations 2-5) are the major site of airway resistance, not the smallest airways, because of their summated smaller cross-sectional area compared to the numerous terminal bronchioles in parallel.

High‑Yield Points - ⚡ Biggest Takeaways

• Inspiration: active process (diaphragm, external intercostals).
• Expiration: normally passive (elastic recoil); forced uses internal intercostals, abdominals.
• Intrapleural pressure: always negative, crucial for lung expansion.
• Surfactant (DPPC): ↓ surface tension, prevents alveolar collapse, ↑ compliance. Deficiency causes RDS.
• Lung compliance: ↑ in emphysema, ↓ in fibrosis/edema.
• Airway resistance: highest in medium-sized bronchi.
• Work of breathing: ↑ with ↓ compliance or ↑ resistance.