Chapter 4: Respiratory System — Gas Exchange and Breathing Mechanics

The respiratory system is responsible for supplying oxygen to the blood and expelling carbon dioxide. Respiratory diseases — including COPD, asthma, and pneumonia — are among the leading causes of morbidity worldwide.

Airway Anatomy

Upper airway: nose → nasopharynx → oropharynx → larynx Lower airway: trachea → main bronchi → lobar bronchi → segmental bronchi → bronchioles → terminal bronchioles → respiratory bronchioles → alveolar ducts → alveoli

The alveoli are the functional units of gas exchange. They are surrounded by a dense capillary network. The alveolar-capillary membrane is extremely thin (~0.5 µm), optimized for rapid diffusion. The lungs contain approximately 480 million alveoli in an adult.

Surfactant, produced by type II pneumocytes, reduces alveolar surface tension and prevents collapse (atelectasis). Deficiency in premature neonates causes neonatal respiratory distress syndrome (NRDS).

Gas Exchange and Breathing Mechanics

Gas exchange follows Fick’s law of diffusion: rate ∝ (surface area × partial pressure difference) / membrane thickness.

• Inspired air: ~21% O₂ (pO₂ ~160 mmHg)
• Alveolar pO₂: ~100 mmHg; arterial pO₂: ~95 mmHg
• Venous pCO₂: ~46 mmHg; alveolar pCO₂: ~40 mmHg

Ventilation mechanics:

• Inspiration: diaphragm and external intercostals contract → thoracic volume increases → pressure falls → air flows in (negative pressure breathing)
• Expiration: at rest, passive recoil; forced expiration uses internal intercostals and abdominal muscles

Spirometry

Spirometry measures lung volumes and flows to classify respiratory disease:

Measurement	Normal Value	Significance
FVC (Forced Vital Capacity)	~80% predicted	Total air exhaled forcefully
FEV₁ (Forced Expiratory Volume in 1s)	~80% predicted	Air exhaled in first second
FEV₁/FVC ratio	≥0.70	Key diagnostic ratio
TLC (Total Lung Capacity)	~6 L (male)	All air in lungs
RV (Residual Volume)	~1.2 L	Air remaining after maximal exhalation

Obstructive pattern (COPD, asthma): ↓FEV₁/FVC, ↑RV (air trapping) Restrictive pattern (pulmonary fibrosis, obesity): ↓FVC, normal or ↑FEV₁/FVC, ↓TLC

COPD vs Asthma

Feature	COPD	Asthma
Age of onset	Usually >40	Often childhood/young adult
Primary cause	Smoking (>90%)	Allergens, triggers, genetics
Reversibility	Mostly irreversible	Largely reversible
Spirometry	Fixed obstruction	Reversible after bronchodilator
Inflammation	Neutrophilic, macrophage	Eosinophilic (type 2)
Key drugs	LABA, LAMA, ICS	ICS, SABA, LABA, biologics

Common Pneumonia Pathogens

Setting	Common Pathogens
Community-acquired	S. pneumoniae, H. influenzae, Mycoplasma (atypical)
Hospital-acquired (HAP)	Pseudomonas, Klebsiella, MRSA
Immunocompromised	P. jirovecii (PCP), Aspergillus, CMV

Key Checklist

• Explains alveolar gas exchange using Fick’s law and partial pressures
• Interprets spirometry results to distinguish obstructive from restrictive disease
• Compares the pathophysiology, triggers, and management of COPD vs asthma