Lung Pathology — Must-Know

## Summary This document provides a comprehensive overview of lung pathology, covering normal lung structure, common conditions like atelectasis and ARDS, and detailed explanations of obstructive and restrictive lung diseases. It delves into specific obstructive conditions such as emphysema, chronic bronchitis, asthma, and bronchiectasis, and examines various chronic interstitial lung diseases including idiopathic pulmonary fibrosis, pneumoconioses (coal dust, silica, asbestos), sarcoidosis, hypersensitivity pneumonitis, pulmonary eosinophilia, and smoking-related interstitial diseases. The text also addresses pulmonary vascular diseases, focusing on pulmonary embolism, pulmonary hypertension, and diffuse alveolar haemorrhage syndromes. Finally, it details a wide range of pulmonary infections, from community-acquired bacterial and viral pneumonias to hospital-acquired, aspiration, and abscess formations, with specific sections on tuberculosis, nontuberculous mycobacteria, fungal infections, pneumonia in immunocompromised hosts, and pulmonary disease in HIV. ## Key Points - **Acinus**: The functional unit of the lung distal to the terminal bronchiole, responsible for gas exchange. - **Atelectasis**: Loss of lung volume due to inadequate expansion of air spaces, potentially reversible except for contraction atelectasis. - **ARDS (Acute Respiratory Distress Syndrome)**: Severe lung injury characterized by diffuse alveolar damage, hypoxemia, and non-cardiogenic pulmonary edema. - **Obstructive Lung Disease**: Characterized by increased resistance to airflow, leading to a decreased FEV1/FVC ratio (e.g., emphysema, asthma). - **Restrictive Lung Disease**: Characterized by reduced lung expansion, leading to a reduced FVC with a near-normal FEV1/FVC ratio (e.g., IPF, sarcoidosis). - **Emphysema**: Permanent enlargement of air spaces distal to terminal bronchioles with alveolar wall destruction. - **Chronic Bronchitis**: Defined clinically by a persistent productive cough for at least 3 months in 2 consecutive years, marked by mucus hypersecretion. - **Asthma**: Chronic inflammatory airway disorder with reversible obstruction, bronchial inflammation, and hyperreactivity, driven by TH2 cells and eosinophils. - **Bronchiectasis**: Permanent dilation of bronchi and bronchioles due to destruction of smooth muscle and elastic tissue, often secondary to persistent infection or obstruction. - **Idiopathic Pulmonary Fibrosis (IPF)**: A chronic, progressive interstitial lung disease of unknown etiology characterized by Usual Interstitial Pneumonia (UIP) pattern. - **Pneumoconioses**: Lung disorders caused by inhalation of mineral dusts, most commonly coal dust, silica, and asbestos. - **Sarcoidosis**: A multisystem disease characterized by noncaseating granulomatous inflammation in various organs. - **Hypersensitivity Pneumonitis**: An immunologically mediated inflammatory lung disease affecting alveoli, triggered by inhaled antigens. - **Pulmonary Embolism**: Blockage of pulmonary arteries, most commonly by thrombi from deep leg veins. - **Pulmonary Hypertension**: Elevated pulmonary arterial pressure, classified into five groups based on underlying causes. - **Diffuse Alveolar Haemorrhage Syndromes**: Conditions involving bleeding into the alveolar spaces, such as Goodpasture syndrome and Granulomatosis with Polyangiitis (GPA). - **Tuberculosis**: A communicable chronic granulomatous disease caused by Mycobacterium tuberculosis, characterized by granuloma formation and potential for cavitation. - **Pneumonia in Immunocompromised Host**: Increased susceptibility to opportunistic infections like Pneumocystis jiroveci, CMV, fungi, and atypical bacteria. ## Detailed Notes ### 1. Normal Lung Structure **Key concepts:** Airway anatomy · Acinus · Alveolar wall components · Cell types - The respiratory system develops as an outgrowth from the ventral wall of the foregut. - The right lung has 3 lobes and 3 main bronchi; the left lung has 2 lobes and 2 main bronchi. - Airways progress from bronchi to bronchioles (lacking cartilage and submucosal glands) to terminal bronchioles, and finally to the acinus. - The **acinus** is the part of the lung distal to the terminal bronchiole, consisting of respiratory bronchioles, alveolar ducts, alveolar sacs, and alveoli, which are the primary sites of gas exchange. **Alveolar Wall Components (Blood to Air):** - Capillary endothelium and its basement membrane. - Pulmonary interstitium, containing elastic fibers, collagen, fibroblasts, smooth muscle cells, mast cells, and mononuclear cells. - Alveolar epithelium: - **Type I pneumocytes**: Flat, plate-like cells covering 95% of the alveolar surface. - **Type II pneumocytes**: Rounded cells that synthesize surfactant and are crucial for repair after type I pneumocyte injury. - Alveolar macrophages: These cells reside freely in the alveolar space and can contain carbon particles in urban dwellers. ### 2. Atelectasis (Collapse) **Con