Lung Pathology

## Normal Lung Structure ### Anatomy and Development * **Development**: The respiratory system develops as an outgrowth from the ventral wall of the foregut. * **Lobes**: The right lung has 3 lobes; the left lung has 2 lobes. * **Airways**: Bronchi lead to bronchioles (lacking cartilage and submucosal glands), then to terminal bronchioles, and finally to the acinus. * **Acinus**: The functional unit distal to the terminal bronchiole, composed of respiratory bronchioles, alveolar ducts, alveolar sacs, and alveoli. ### Alveolar Wall and Cell Types * **Alveolar Wall Components**: Capillary endothelium, basement membrane, and pulmonary interstitium (containing elastic fibers, collagen, fibroblasts, and mast cells). * **Type I Pneumocytes**: Flat cells covering 95% of the alveolar surface. * **Type II Pneumocytes**: Rounded cells that synthesize surfactant and act as progenitor cells for repair after injury. * **Alveolar Macrophages**: Phagocytic cells located within the alveolar space. ## Atelectasis (Collapse) Definition: Loss of lung volume caused by inadequate expansion of air spaces, resulting in ventilation-perfusion imbalance. ### Classification 1. **Resorption Atelectasis**: Caused by total obstruction of an airway (e.g., mucus plugs, foreign bodies, tumors). Air distal to the obstruction is absorbed. 2. **Compression Atelectasis**: Caused by accumulation of fluid, blood, or air in the pleural cavity (e.g., pleural effusion, pneumothorax). 3. **Contraction Atelectasis (Cicatrization)**: Caused by local or diffuse fibrosis that prevents full expansion. This form is irreversible. ## Acute Respiratory Distress Syndrome (ARDS) Definition: Acute respiratory failure characterized by bilateral opacities on imaging, not explained by cardiac failure. It involves Diffuse Alveolar Damage (DAD). ### Pathogenesis * Triggered by pneumonia, sepsis, aspiration, or trauma. * **Mechanism**: Endothelial and epithelial injury leads to vascular leakiness and loss of surfactant. * **Neutrophil Role**: Macrophages release IL-8, recruiting neutrophils that release reactive oxygen species and proteases, damaging the alveolar-capillary membrane. ### Morphology * **Acute Phase**: Lungs are heavy and firm. Characterized by hyaline membranes (fibrin-rich edema and necrotic debris) lining alveolar ducts. * **Organizing Stage**: Proliferation of Type II pneumocytes and interstitial fibrosis. ## Obstructive vs. Restrictive Lung Disease | Feature | Obstructive (e.g., COPD, Asthma) | Restrictive (e.g., IPF, Sarcoidosis) | | :--- | :--- | :--- | | **Mechanism** | Increased resistance to airflow | Reduced lung expansion/compliance | | **FVC** | Normal or slightly decreased | Significantly reduced | | **FEV1** | Significantly decreased | Reduced proportionately | | **FEV1/FVC** | Decreased (<0.7) | Normal or near normal | ## Obstructive Lung Diseases ### Emphysema * **Definition**: Permanent enlargement of air spaces distal to terminal bronchioles with wall destruction without significant fibrosis. * **Types**: * **Centriacinar**: Affects proximal acinus; associated with smoking; upper lobes. * **Panacinar**: Affects entire acinus; associated with alpha-1-antitrypsin deficiency; lower lobes. * **Pathogenesis**: Protease-antiprotease imbalance and oxidative stress. ### Chronic Bronchitis * **Clinical Definition**: Persistent productive cough for 3 months in 2 consecutive years. * **Morphology**: Hypertrophy of mucus-secreting glands (Reid index > 0.4) and goblet cell metaplasia. ### Asthma * **Hallmarks**: Reversible airway obstruction, eosinophilic inflammation, and smooth muscle hypertrophy. * **Atopic Asthma**: IgE-mediated (Type I hypersensitivity) driven by TH2 cells (IL-4, IL-5, IL-13). * **Morphology**: Curschmann spirals and Charcot-Leyden crystals in sputum. ### Bronchiectasis * **Definition**: Permanent dilation of bronchi due to destruction of muscle and elastic tissue from chronic necrotizing infections. * **Associations**: Cystic fibrosis, Kartagener syndrome, and bronchial obstruction. ## Chronic Interstitial (Restrictive) Lung Diseases ### General Features * Reduced lung compliance and "honeycomb lung" in end-stage disease. * **Idiopathic Pulmonary Fibrosis (IPF)**: Progressive bilateral interstitial fibrosis with a "usual interstitial pneumonia" (UIP) pattern. * **Pneumoconioses**: Lung injury due to inhalation of mineral dusts (Coal, Silica, Asbestos). * **Sarcoidosis**: Multisystem disease characterized by noncaseating granulomas and elevated ACE levels. * **Hypersensitivity Pneumonitis**: Immunologically mediated reaction to inhaled organic antigens (e.g., Farmer's lung).