The Respiratory System: Structure, Function, and Histology

The Respiratory System

Overview and Functions

The respiratory system is essential for gas exchange, providing oxygen to body tissues and removing carbon dioxide. It also plays roles in sound production, olfaction, acid-base balance, blood pressure regulation, and homeostasis.

• Gas Exchange: Supplies oxygen (O2) to tissues and removes carbon dioxide (CO2).

• Sound Production: Enables phonation (speech) via the larynx.

• Olfaction: Assists in the sense of smell.

• Acid-Base Balance: Regulates blood pH by controlling CO2 elimination.

• Blood Pressure Regulation: Involved in the generation of angiotensin II for blood pressure and fluid homeostasis.

Components and Zones of the Respiratory System

The respiratory system is divided into anatomical regions and functional zones:

Main Components

• Nasal cavity

• Pharynx

• Larynx

• Trachea

• Primary bronchi

• Secondary bronchi

• Tertiary bronchi

• Bronchioles

• Terminal bronchioles

• Respiratory bronchioles

• Alveolar duct

• Alveolar sac

• Alveolus

Functional Zones

• Conducting Zone (Anatomical Dead Space): Includes nasal cavity to terminal bronchioles; functions to filter, warm, and humidify air (approx. 150 mL volume).

• Respiratory Zone: Includes respiratory bronchioles, alveolar ducts, sacs, and alveoli; site of gas exchange.

Histology: Respiratory Mucosa

The respiratory mucosa is a specialized mucous membrane lining much of the respiratory tract, consisting of an epithelium and underlying lamina propria (areolar connective tissue). The type of epithelium changes along the tract to suit functional needs.

• Nasal cavity and nasopharynx: Pseudostratified ciliated columnar epithelium with mucous-producing glands and goblet cells (anterior vestibule is stratified squamous).

• Oropharynx and laryngopharynx: Non-keratinized stratified squamous epithelium (protects against abrasion from food).

• Trachea through large bronchioles: Pseudostratified ciliated columnar epithelium with mucous glands and goblet cells.

• Smaller bronchioles to terminal and respiratory bronchioles: Gradual change to simple cuboidal epithelium with few cilia; no mucous glands or goblet cells.

• Alveoli: Simple squamous epithelium (Type I pneumocytes) for gas exchange; Type II pneumocytes (septal cells) produce surfactant to reduce surface tension.

Key Terms

• Lamina propria: Loose connective tissue supporting the epithelium.

• Goblet cells: Mucus-secreting cells found in the respiratory epithelium.

• Surfactant: A phospholipoprotein that reduces surface tension in alveoli, preventing collapse.

Example: Epithelial Changes Along the Respiratory Tract

• Air entering through the nose encounters pseudostratified ciliated columnar epithelium, which traps and moves particles.

• As air moves deeper, the epithelium transitions to simple squamous in the alveoli, optimizing for gas diffusion.

Region	Epithelium Type	Special Features
Nasal cavity, nasopharynx	Pseudostratified ciliated columnar	Mucous glands, goblet cells
Oropharynx, laryngopharynx	Non-keratinized stratified squamous	Protection from abrasion
Trachea, large bronchioles	Pseudostratified ciliated columnar	Mucous glands, goblet cells
Terminal/respiratory bronchioles	Simple cuboidal	Few cilia, no mucous glands
Alveoli	Simple squamous (Type I & II)	Type II cells produce surfactant

Additional info: The mucociliary escalator is a defense mechanism where cilia move mucus and trapped particles out of the respiratory tract.