Respiratory System Functions: Gas Exchange, pH Regulation, and Phonation

Three Primary Functions

1. Gas exchange (external respiration):
   • O₂: Alveoli (PAO₂ ~100 mmHg) → pulmonary capillaries → Hb binding → tissues
   • CO₂: Tissues → blood (dissolved, bicarbonate, carbamino) → alveoli (PACO₂ ~40 mmHg) → exhaled
   • Driven by partial pressure gradients; Fick's law of diffusion
2. Acid-base balance (pH regulation):
   • CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻ (carbonic anhydrase)
   • ↑ ventilation → ↓ PaCO₂ → ↑ pH (respiratory alkalosis)
   • ↓ ventilation → ↑ PaCO₂ → ↓ pH (respiratory acidosis)
   • Rapid compensation (minutes–hours); renal compensation (days)
3. Phonation (voice production):
   • Expiratory airflow through adducted vocal folds → Bernoulli effect → vibration
   • Pitch: vocal fold tension (cricothyroid), length, mass
   • Quality: supraglottic articulation, resonance
   • Loudness: subglottic pressure, glottal closure

Additional Functions

• Olfaction (nasal airflow)
• Pulmonary defense (mucociliary escalator, macrophages, IgA, cough reflex)
• Metabolic: ACE (angiotensin conversion), prostaglandin synthesis, filter microemboli
• Thermoregulation (panting, heat exchange)