Chapter 23, Lecture 3

Intrapleural Pressure ( P_{ip} )

  • Pleural cavity between parietal & visceral pleura contains thin serous fluid ➔ creates surface tension
  • Surface tension forms a partial vacuum ➔ P{ip} always < atmospheric pressure ( P{atm}=760\,\text{mmHg} )
    • At rest: P_{ip}\approx 756\,\text{mmHg} ( −4 mmHg )
  • Negative P_{ip} mechanically couples lung to thoracic wall; lungs follow chest movements

Pressure Changes During Quiet Breathing

  • Boyle’s law: P \propto \dfrac{1}{V} (in closed system)

Inhalation

  • Muscles: diaphragm + external intercostals contract ➔ thoracic volume ↑
  • Parietal pleura pulled outward ➔ lungs & alveoli expand
  • Alveolar pressure falls to P_{alv}=758\,\text{mmHg}
  • Gradient: 760 \rightarrow 758\,\text{mmHg} ➔ air flows in
  • P_{ip} remains negative
  • Deep inhalation recruits scalenes & sternocleidomastoid

Exhalation

  • Muscles relax; elastic recoil shrinks thoracic volume
  • Alveolar volume ↓ ➔ P_{alv}=762\,\text{mmHg}
  • Gradient: 762 \rightarrow 760\,\text{mmHg} ➔ air flows out
  • Forced exhalation uses internal intercostals & abdominal muscles

Factors Affecting Ventilation (besides pressure gradient)

  1. Surface tension of alveolar fluid
    • Water molecules tend to collapse alveoli
    • Type II cells secrete surfactant ➔ lowers tension, keeps alveoli open
    • Surfactant deficiency → neonatal respiratory distress syndrome
  2. Lung compliance (ease of expansion)
    • Depends on elasticity + surface tension
    • Low compliance: scar tissue, edema, surfactant lack, muscle paralysis, thoracic arthritis
  3. Airway resistance (diameter of bronchioles)
    • Bronchoconstriction ↑ resistance, ↓ airflow (parasymp, cold air, inflammation, mucus)
    • Bronchodilation ↓ resistance, ↑ airflow (symp, epinephrine → EpiPen)
    • Airflow ∝ \dfrac{\Delta P}{R}

Clinical Connection

  • Pneumothorax = air in pleural cavity (trauma or spontaneous)
    • Eliminates negative P_{ip} ➔ lung collapses
  • COPD/obstructive disease: ↑ airway resistance; patients expend up to 25 % of energy on breathing