The respiratory tract moves air in and out of the lungs while filtering, warming, and humidifying it along the way. It's split into upper and lower portions, and further divided into a conducting zone (which moves air) and a respiratory zone (where gas exchange actually happens). Understanding the anatomy of each region is essential for everything that follows in this unit, from ventilation mechanics to gas transport.
Respiratory Tract Structures and Regions
Upper and Lower Respiratory Tract
The respiratory tract divides into two major regions based on location:
- The upper respiratory tract includes the nose, nasal cavity, paranasal sinuses, and pharynx. Its primary job is to condition incoming air by filtering particles, warming it to body temperature, and adding moisture.
- The lower respiratory tract includes the larynx, trachea, bronchi, and lungs. This is where air is conducted deeper into the system and where gas exchange ultimately takes place.
Nasal Cavity and Pharynx
The nasal cavity is the first major conditioning station for inspired air. It's lined with respiratory epithelium and contains three shelf-like projections called conchae (also called turbinates). The conchae increase the surface area inside the nasal cavity, forcing air to swirl over the warm, moist mucosal lining. This turbulent airflow is what makes filtering, warming, and humidifying so effective.
The pharynx is a muscular tube that connects the nasal and oral cavities to the larynx and esophagus. It has three subdivisions:
- Nasopharynx: sits posterior to the nasal cavity; functions as an airway and contributes to resonance during speech
- Oropharynx: sits posterior to the oral cavity; serves as a shared passageway for both air and food
- Laryngopharynx: the most inferior portion; connects to both the larynx (airway) and the esophagus (digestive tract)
Larynx, Trachea, and Bronchi
The larynx (voice box) is a cartilaginous structure that connects the pharynx to the trachea. It houses the vocal cords, which vibrate to produce sound. The epiglottis, a flap of elastic cartilage attached to the larynx, folds over the airway opening during swallowing to prevent food and liquid from entering the trachea.
The trachea (windpipe) extends from the larynx down to where it splits into the two primary bronchi. It's reinforced by C-shaped cartilaginous rings that keep the airway open. The rings are C-shaped (open on the posterior side) so the esophagus, which sits directly behind the trachea, can expand during swallowing. The trachea is lined with pseudostratified ciliated columnar epithelium, which continuously sweeps mucus and trapped debris upward.
The bronchi are the main branches of the trachea. They enter the lungs and continue to divide into progressively smaller bronchioles. As the airways branch and get smaller, the cartilage support gradually disappears and smooth muscle becomes more prominent. The smallest bronchioles terminate at clusters of alveoli, the primary sites of gas exchange.
Functions of Respiratory System Components
Nose, Pharynx, and Larynx Functions
- The nose filters, warms, and humidifies inspired air. It also houses the olfactory epithelium in the roof of the nasal cavity, which detects smell.
- The pharynx serves as a shared passageway for air and food. The nasopharynx and oropharynx also contribute to speech resonance.
- The larynx has two critical protective and functional roles: the epiglottis prevents food and liquid from entering the lower airway, and the vocal cords produce sound through vibration.
Trachea, Bronchi, and Lungs Functions
- The trachea conducts air from the larynx to the bronchi. Its ciliated epithelium continuously moves mucus and trapped particles upward toward the pharynx for removal.
- The bronchi and bronchioles conduct air from the trachea to the alveoli. Their smooth muscle walls can adjust airway diameter through bronchoconstriction (narrowing) and bronchodilation (widening), regulating airflow to different lung regions.
- The lungs are the primary organs of respiration. Gas exchange occurs across the walls of the alveoli, which are surrounded by dense capillary networks.
- The right lung has three lobes (upper, middle, and lower), while the left lung has two lobes (upper and lower). The left lung is slightly smaller to accommodate the heart.
- Both lungs are enclosed by the pleura, a double-layered serous membrane. The visceral pleura covers the lung surface, and the parietal pleura lines the thoracic wall. A thin layer of serous fluid between them reduces friction during breathing.
Respiratory Epithelium Histology
Cell Types and Functions
Respiratory epithelium is pseudostratified ciliated columnar epithelium that lines most of the respiratory tract, from the nasal cavity down through the bronchioles. "Pseudostratified" means the cells appear to be layered but all actually contact the basement membrane. This epithelium contains several specialized cell types:
- Ciliated cells have motile cilia on their apical surface that beat in a coordinated wave, pushing mucus and trapped debris upward toward the pharynx. This mechanism is called the mucociliary escalator. Once the mucus reaches the pharynx, it's either swallowed or expectorated.
- Goblet cells secrete mucus, which traps inhaled particles, pathogens, and dust while keeping the airway surface moist.
- Brush cells are chemosensitive cells that detect irritants in the airway. They may help regulate the mucociliary escalator in response to harmful substances.
- Basal cells are stem cells that sit along the basement membrane. They divide and differentiate to replace damaged or aging epithelial cells, keeping the lining intact.
- Small granule cells (neuroendocrine cells) secrete bioactive amines and peptides that may help regulate airway function and local immune responses.
Submucosal Glands and Innate Immunity
Beneath the respiratory epithelium, submucosal glands secrete additional mucus and antimicrobial substances. Together with goblet cells, they produce the mucus layer that traps inhaled particles and pathogens.
The mucociliary escalator then removes this contaminated mucus from the airway. On top of physical trapping, the submucosal glands secrete antimicrobial molecules like lysozyme (which breaks down bacterial cell walls) and defensins (which punch holes in microbial membranes). These substances are part of the respiratory tract's innate immune defense, neutralizing pathogens before they can cause infection.
Conducting vs. Respiratory Zones
Conducting Zone Structure and Function
The conducting zone includes all the structures that transport air but do not participate in gas exchange: the nose, pharynx, larynx, trachea, bronchi, and bronchioles (down to and including the terminal bronchioles).
This zone is lined with pseudostratified ciliated columnar epithelium, which filters, warms, and humidifies inspired air while the mucociliary escalator removes debris. Because the conducting zone lacks alveoli, no gas exchange occurs here. The volume of air that fills these passages without reaching the respiratory zone is called anatomical dead space (approximately 150 mL in an average adult).
Respiratory Zone Structure and Function
The respiratory zone is where gas exchange actually happens. It includes the respiratory bronchioles, alveolar ducts, and alveoli.
Unlike the conducting zone, the respiratory zone is lined with simple squamous epithelium, which is extremely thin (as little as 0.2 µm). This thinness is critical because gases must diffuse across the alveolar wall and into the surrounding capillaries. A thick epithelium would slow diffusion dramatically.
The alveoli are organized into alveolar sacs, which are grape-like clusters that share a common opening. The alveolar walls contain two key cell types:
- Type I alveolar cells: thin, flat squamous cells that make up most of the alveolar surface area. Gas exchange occurs across these cells.
- Type II alveolar cells: cuboidal secretory cells that produce pulmonary surfactant, a phospholipid mixture that reduces surface tension inside the alveoli and prevents them from collapsing during exhalation.
Transition Between Conducting and Respiratory Zones
The shift from conducting to respiratory zone happens gradually:
- Terminal bronchioles are the last purely conducting structures. They're the smallest airways that contain no alveoli.
- Respiratory bronchioles branch off from terminal bronchioles and have a small number of alveoli scattered in their walls. This is where the respiratory zone begins.
- Alveolar ducts follow the respiratory bronchioles and are almost entirely lined with alveoli.
- Alveolar sacs are the most distal structures, consisting entirely of clustered alveoli. These are the primary sites of gas exchange in the lungs.