Pharmacy - Histology of Respiratory System

The Respiratory System

• Consists of paired lungs and a series of air passages that lead to the lungs
• Performs three principal functions: air conduction, air filtration, and gas exchange (respiration)
• Gas exchange occurs in the alveoli

Conducting Portion

• Includes nasal cavities, larynx, trachea, and paired main bronchi
• Located outside and within the lungs
• Trachea is a flexible air tube that extends from the larynx to the thorax and divides into paired main bronchi in the mediastinum
• Paired main bronchi enter the root of the right or left lung
• Within the lungs, the main bronchi undergo extensive branching to give rise to bronchioles

The Respiratory Portion

• Includes respiratory bronchioles, alveolar ducts, and alveoli
• Where gas exchange occurs

Tracheal Wall Structure

• Composed of four definable layers:
  • Mucosa: ciliated, pseudostratified epithelium and an elastic fiber-rich lamina propria
  • Submucosa: slightly denser connective tissue than the lamina propria
  • Cartilaginous layer: C-shaped hyaline cartilages that provide a supporting structure and prevent collapse of the tracheal lumen
  • Adventitia: connective tissue that binds the trachea to adjacent structures

Tracheal Epithelium

• Composed of ciliated columnar cells and mucous (goblet) cells
• Ciliated cells are the most numerous and provide a coordinated sweeping motion of the mucous coat toward the pharynx
• Mucous cells are interspersed among the ciliated cells and produce mucinogen

Bronchi and Bronchioles

• Trachea divides into two main bronchi, which further divide into secondary bronchi
• Right bronchus is wider and shorter than the left
• Left lung is divided into two lobes, and the right lung is divided into three lobes
• Each main bronchus divides into secondary bronchi, which then branch into smaller bronchi
• Bronchioles are air-conducting ducts that measure 1 mm or less in diameter
• Terminal bronchioles give rise to respiratory bronchioles

Respiratory Bronchioles and Alveoli

• Respiratory bronchioles are involved in both air conduction and gas exchange
• Alveolar ducts are elongated airways that have almost no walls, only alveoli
• Alveoli are the terminal air spaces of the respiratory system and are the actual sites of gas exchange between air and blood
• Each alveolus is surrounded and separated from one another by a thin connective tissue layer that contains blood capillaries (alveolar septum)
• Alveolar epithelium is composed of type I and II alveolar cells
• Type I alveolar cells are extremely thin, squamous cells that are not capable of cell division
• Type II alveolar cells are cuboidal cells that are interspersed among the type I cells and are progenitor cells for type I alveolar cells