Chapter 19: Respiratory System

The respiratory system is functionally divided into a conducting portion responsible for filtering, conditioning, and moving air, and a respiratory portion where crucial gas exchange takes place. Developing from the foregut endoderm, the system begins at the nasal cavities, which are composed of the vestibule, the main respiratory region, and the specialized olfactory region. The respiratory region is lined by pseudostratified columnar epithelium, often called respiratory mucosa, which features abundant ciliated cells that create the mucociliary escalator—a vital protective mechanism for moving trapped particles towards the pharynx—alongside mucus-secreting goblet cells and sensory brush cells. Distinctively, the olfactory region, responsible for the sense of smell, lacks goblet cells and contains specialized bipolar olfactory receptor neurons whose axons bundle to form the first cranial nerve. Olfactory receptor function is supported by columnar supporting cells and the serous secretion of Bowman glands, which solubilizes odorants and prevents dehydration. Air passes through the pharynx and into the larynx, an organ of phonation reinforced by hyaline and elastic cartilages, housing the vocal folds which control airflow and vibrate to produce sound. The trachea, a flexible tube, is held open by C-shaped hyaline cartilage rings and features a prominent, thick basement membrane underlying its respiratory epithelium. As airways branch into intrapulmonary bronchi, the cartilage rings are replaced by irregular plates and a complete layer of circumferential smooth muscle. Further narrowing creates bronchioles, which lack cartilage and glands entirely. The smallest conducting tubes, terminal bronchioles, transition to a cuboidal epithelium populated by Club cells (formerly Clara cells), nonciliated cells that produce a surface-active lipoprotein that prevents luminal collapse during expiration. Gas exchange begins in the respiratory bronchioles and proceeds into the terminal air spaces, the alveoli. The alveolar wall forms the air-blood barrier, consisting of extremely thin Type I alveolar cells (pneumocytes), which cover 95% of the surface, and capillary endothelial cells, often joined by a fused basal lamina. Crucially, Type II alveolar cells (septal cells) are cuboidal secretory cells that produce and secrete surfactant (via lamellar bodies), a phospholipid mixture that significantly reduces surface tension necessary for lung inflation and prevents alveolar collapse. The lung is protected by alveolar macrophages (dust cells) which remove particulate matter from the air spaces. Pathological conditions include chronic diseases like asthma (airway inflammation and hyperplastic smooth muscle), cystic fibrosis (defective chloride channel leading to highly viscous mucus and impaired clearance), and emphysema (destruction and permanent enlargement of alveolar air spaces).