Summary
Highlights
Type II cuboidal cells in the alveoli secrete surfactant, which reduces surface tension and prevents alveolar collapse. Alveoli also contain macrophages that engulf inhaled particles that have bypassed previous defenses, with debris-laden macrophages then cleared by the mucociliary escalator.
The respiratory system's primary role is to exchange oxygen and carbon dioxide. This gas exchange occurs in the respiratory division within the lungs, while other parts like the nose, pharynx, larynx, trachea, bronchi, and bronchioles form the conducting division, acting as air passageways.
The nasal cavity is lined with a ciliated mucous membrane that traps inhaled particles and moves debris towards the throat for swallowing. Lysozyme, lymphocytes, and antibodies in the mucus provide protection against pathogens.
Nasal conchae (turbinates) increase the contact surface with inhaled air, warming, moistening, and cleansing it. The roof of the nasal cavity contains olfactory nerve cells responsible for the sense of smell.
Air makes a 90-degree turn in the pharynx, which helps trap large dust particles. The pharynx also houses tonsils, immunocompetent tissues that respond to inhaled pathogens. It serves as a pathway for both air to the lungs and food/drink to the esophagus.
The larynx is crucial in preventing food or drink from entering the lungs. During swallowing, the epiglottis guards the laryngeal opening, flipping over to direct substances to the esophagus, while vocal folds also close to protect the airway.
Air passes from the larynx to the trachea (windpipe), which divides into two primary bronchi leading to the lungs. These bronchi further branch into smaller bronchi and bronchioles, forming a bronchial tree with millions of airways. The smooth muscle in these airways constricts or dilates to regulate airflow.
The larynx, trachea, and bronchial tree are lined with ciliated columnar epithelium that produces mucus. This mucociliary escalator traps inhaled particles and moves them towards the throat to be swallowed, effectively cleansing the airways.
Terminal bronchioles branch into respiratory bronchioles, marking the start of the respiratory division. These end in alveoli, microscopic air sacs surrounded by blood capillaries, where gas exchange occurs. Type I cells in the alveolar wall facilitate rapid gas diffusion, with oxygen moving into the blood and carbon dioxide moving out.