Chapter 54: Pleura, Lungs, Trachea & Bronchi

Pleura, Lungs, Trachea & Bronchi details the anatomy and physiology of the organs vital for gas exchange, the lungs, along with the protective pleura and the conducting airways: the trachea and bronchi. Respiration is initiated by muscles and the respiratory diaphragm increasing intrathoracic volume, which causes a negative pressure in the pleural cavity that draws air into the lungs and facilitates gaseous diffusion at the alveoli. The lungs maintain crucial homeostasis by efficiently transferring oxygen while ensuring slightly inefficient carbon dioxide transfer, maintaining its blood pressure significantly higher (38–42 mmHg) than the atmosphere to regulate hydrogen ion concentration. Each lung is enveloped by the double-layered serous membrane, the pleura, consisting of the adhering visceral pleura and the parietal pleura, which lines the thoracic wall, diaphragm, and mediastinum; the thin pleural fluid between these layers permits low-friction movement during breathing. Lung morphology typically presents as a halved cone, featuring an apex, a base (diaphragmatic surface), and borders; the left lung is divided into superior and inferior lobes by an oblique fissure, featuring a prominent cardiac notch and the lingula, while the right lung has three lobes (superior, middle, inferior) separated by oblique and horizontal fissures. Air passes through the trachea, a fibromuscular tube reinforced by 16–20 incomplete cartilaginous rings, before bifurcating into the right and left main bronchi. The right main bronchus is wider, shorter, and more vertical than the longer, narrower left main bronchus, which passes inferior to the aortic arch. These main bronchi divide into lobar and subsequently segmental bronchi (B1-B10), supplying distinct, functionally independent bronchopulmonary segments (S1-S10). Microstructurally, the air-exchange units, the alveoli (totaling 300–500 million), are lined predominantly by Type I pneumocytes (for gas diffusion), alongside Type II pneumocytes, which are responsible for secreting pulmonary surfactant to reduce surface tension and maintain alveolar patency. The lungs receive dual circulation: the pulmonary arteries deliver deoxygenated blood for oxygenation, while the systemic bronchial arteries provide nutritional support to the bronchial walls and connective tissues. Defensive mechanisms include the mucociliary escalator, lymphoid tissues, and the cough reflex, which expels materials triggered by vagal afferents. Clinical relevance is highlighted in conditions like pneumothorax (air accumulation) and pulmonary sequestration (anomalous systemic arterial supply to lung tissue).