BREATHING & EXCHANGE OF GASES
├── RESPIRATORY SYSTEM ANATOMY
│   ├── Conducting Zone (no gas exchange)
│   │   ├── External nares → Nasal cavity (warms, moistens, filters)
│   │   ├── Pharynx → Larynx (vocal cords)
│   │   ├── Trachea (C-shaped cartilaginous rings)
│   │   └── Primary → Secondary → Tertiary bronchi → Bronchioles
│   ├── Respiratory Zone (gas exchange)
│   │   ├── Respiratory bronchioles
│   │   ├── Alveolar ducts
│   │   └── Alveoli (functional units, ~700 million, ~70 $m^{2}$)
│   └── Lung structure
│       ├── Right lung: 3 lobes
│       ├── Left lung: 2 lobes (cardiac notch)
│       └── Double pleural membranes (intrapleural fluid, negative pressure)
│
├── BREATHING MECHANISM
│   ├── Inspiration (ACTIVE)
│   │   ├── Diaphragm contracts (flattens)
│   │   ├── External intercostals lift ribs up & out
│   │   ├── Thoracic volume ↑ → pressure ↓ → air IN
│   │   └── Boyle's Law: P ∝ 1/V
│   └── Expiration
│       ├── Quiet (PASSIVE): elastic recoil → air OUT
│       └── Forced (ACTIVE): internal intercostals + abdominals → air OUT
│
├── LUNG VOLUMES & CAPACITIES
│   ├── Basic volumes: TV(500), IRV(2500–3000), ERV(1000–1100), RV(1100–1200 mL)
│   ├── Capacities: IC=TV+IRV, EC=TV+ERV, FRC=ERV+RV, VC=TV+IRV+ERV, TLC=VC+RV
│   └── RV cannot be measured by spirometry
│
├── GAS EXCHANGE (ALVEOLI)
│   ├── Mechanism: Simple diffusion (partial pressure gradients)
│   ├── Diffusion membrane: 3 layers (alveolar epithelium + basement membrane + capillary endothelium)
│   ├── O2: alveoli pO2=104 → blood pO2=40 (gradient=64 mmHg)
│   └── CO2: blood pCO2=45 → alveoli pCO2=40 (gradient=5 mmHg; CO2 is 20× more soluble)
│
├── GAS TRANSPORT
│   ├── O2 Transport
│   │   ├── 97% oxyhaemoglobin (sigmoid dissociation curve, cooperative binding)
│   │   ├── 3% dissolved in plasma
│   │   └── Bohr effect: right shift (CO2↑, pH↓, temp↑, 2,3-BPG↑) → more O2 released
│   └── CO2 Transport
│       ├── 70% as HCO3– (carbonic anhydrase in RBCs; chloride shift = Hamburger's phenomenon)
│       ├── 23% as carbaminohaemoglobin (binds globin amino groups)
│       └── 7% dissolved in plasma
│
├── REGULATION
│   ├── Medullary rhythmicity centre: initiates breathing rhythm
│   ├── Pneumotaxic centre (pons): limits inspiration duration
│   └── Peripheral chemoreceptors: carotid & aortic bodies detect pO2, pCO2, H+
│
└── DISORDERS
    ├── Asthma: allergic airway narrowing (reversible), expiratory difficulty
    ├── Emphysema: alveolar wall destruction, barrel chest, ↑RV, ↓VC
    ├── Silicosis: silica dust → lung fibrosis, progressive dyspnoea
    └── Asbestosis: asbestos → fibrosis + cancer risk (mesothelioma)