The respiratory tract is a complex system that moves air in and out of our lungs. It's split into upper and lower parts, each with unique structures. These work together to filter, warm, and humidify the air we breathe.

From the nose to the alveoli, each part of the respiratory system has a specific job. The conducting zone moves air, while the respiratory zone handles gas exchange. Understanding this anatomy is key to grasping how we breathe and stay alive.

Respiratory Tract Structures and Regions

Upper and Lower Respiratory Tract

• The respiratory tract divides into the upper respiratory tract (nose, nasal cavity, paranasal sinuses, and pharynx) and the lower respiratory tract (larynx, trachea, bronchi, and lungs)
• The upper respiratory tract structures filter, warm, and humidify inspired air before it reaches the lower respiratory tract

Nasal Cavity and Pharynx

• The nose allows for the passage of air into the nasal cavity, which contains respiratory epithelium and conchae (turbinates) that filter, warm, and humidify inspired air
• The pharynx, a muscular tube, connects the nasal and oral cavities to the larynx and esophagus and divides into the nasopharynx, oropharynx, and laryngopharynx
  • The nasopharynx lies posterior to the nasal cavity and functions in respiratory and speech production
  • The oropharynx lies posterior to the oral cavity and functions in both respiratory and digestive processes
  • The laryngopharynx, the most inferior portion, connects to the larynx and esophagus

Larynx, Trachea, and Bronchi

• The larynx (voice box), a cartilaginous structure, connects the pharynx to the trachea and contains the vocal cords, which vibrate to produce sound
• The trachea, a tubular structure reinforced by C-shaped cartilaginous rings, extends from the larynx to the primary bronchi and contains pseudostratified ciliated columnar epithelium
• The bronchi, the main branches of the trachea, lead into the lungs and further divide into smaller bronchioles, which terminate in clusters of alveoli (primary sites of gas exchange)

Functions of Respiratory System Components

Nose, Pharynx, and Larynx Functions

• The nose filters, warms, and humidifies inspired air and detects olfactory stimuli through the olfactory epithelium in the roof of the nasal cavity
• The pharynx serves as a passageway for both air and food and plays a role in speech production through the nasopharynx and oropharynx
• The larynx protects the lower airway by preventing the entry of food and liquid through the epiglottis and functions in speech production through the vibration of the vocal cords

Trachea, Bronchi, and Lungs Functions

• The trachea conducts air from the larynx to the bronchi, and its ciliated epithelium removes debris and mucus from the airway
• The bronchi and bronchioles conduct air from the trachea to the alveoli, and their smooth muscle regulates airflow through bronchoconstriction and bronchodilation
• The lungs, the primary organs of the respiratory system, facilitate gas exchange between the air and the blood through the alveoli, which are surrounded by capillaries
  • The right lung consists of three lobes (upper, middle, and lower), while the left lung consists of two lobes (upper and lower)
  • The lungs are enclosed by the pleura, a double-layered serous membrane that reduces friction during breathing

Respiratory Epithelium Histology

Cell Types and Functions

• The respiratory epithelium, a type of pseudostratified ciliated columnar epithelium, lines most of the respiratory tract (nasal cavity to bronchioles) and consists of several cell types:
  • Ciliated cells possess multiple motile cilia on their apical surface that beat in a coordinated manner to move mucus and debris towards the pharynx for expectoration or swallowing (mucociliary escalator)
  • Goblet cells secrete mucus, which traps inhaled particles and pathogens and maintains the moisture of the airway surface
  • Brush cells are chemosensitive and may play a role in sensing irritants and regulating the mucociliary escalator
  • Basal cells are stem cells that can differentiate into other cell types to replace damaged or aging cells in the respiratory epithelium
  • Small granule cells (neuroendocrine cells) secrete bioactive amines and peptides that may regulate airway function and local immune responses

Submucosal Glands and Innate Immunity

• The respiratory epithelium also contains submucosal glands that secrete mucus and antimicrobial substances, contributing to the innate immune defense of the respiratory tract
• The mucus secreted by goblet cells and submucosal glands traps inhaled particles, pathogens, and debris, which are then removed by the mucociliary escalator
• Antimicrobial substances secreted by the submucosal glands, such as lysozyme and defensins, help to neutralize and eliminate pathogens in the respiratory tract

Conducting vs Respiratory Zones

Conducting Zone Structure and Function

• The conducting zone transports air from the external environment to the respiratory zone and includes the nose, pharynx, larynx, trachea, bronchi, and bronchioles
• The conducting zone is lined with pseudostratified ciliated columnar epithelium, which filters, warms, and humidifies the inspired air and removes debris and mucus from the airway
• The conducting zone does not participate directly in gas exchange, as it lacks alveoli and has a limited blood supply

Respiratory Zone Structure and Function

• The respiratory zone consists of the structures where gas exchange occurs between the air and the blood, including the respiratory bronchioles, alveolar ducts, and alveoli
• The respiratory zone is lined with simple squamous epithelium, which allows for efficient diffusion of gases between the air and the blood
• The alveoli are the primary sites of gas exchange and are surrounded by a dense network of capillaries that facilitate the exchange of oxygen and carbon dioxide between the air and the blood
  • The alveoli are organized into alveolar sacs, which are clusters of alveoli that share a common opening
  • The alveolar walls consist of type I alveolar cells (squamous epithelial cells) and type II alveolar cells (secretory cells that produce pulmonary surfactant)

Transition between Conducting and Respiratory Zones

• The transition from the conducting zone to the respiratory zone occurs at the level of the terminal bronchioles, which are the smallest branches of the bronchial tree that do not contain alveoli
• The respiratory bronchioles, which follow the terminal bronchioles, contain a small number of alveoli in their walls and represent the beginning of the respiratory zone
• The alveolar ducts and alveolar sacs, which are distal to the respiratory bronchioles, are entirely lined with alveoli and are the primary sites of gas exchange in the lungs