Histology of the Respiratory System

Histology Study Notes

• Sheet number: 18+19
• Written by: Mohammad Al-khaliyl and Nadine Dababseh
• Edited by: Razan Al.khaizaran, Noor Hamadneh, and Jeyad Alkhzouz
• Doctor: Ghada Abu el Ghanam
• Year: 2024

Respiratory System

• Structurally divided into two sections: upper and lower respiratory
• Upper respiratory system: nose, nasal cavity, pharynx and associated structures.
• Lower respiratory system: larynx, trachea, bronchi, and lungs.
• Functionally divided into two sections: the conducting and respiratory zones.
• Conducting zone: series of tubes that filter, warm, and humidify the air while moving it into the lungs. These include the nose, nasal cavity, pharynx, larynx, trachea, bronchi, bronchioles to the terminal bronchioles.
• Respiratory zone: consists of tubes inside the lungs for gas exchange. Includes the respiratory bronchioles, alveolar ducts, alveolar sacs, and alveoli.

Respiratory Cells

• Respiratory epithelium is a pseudostratified columnar cells with goblet cells.
• Cell types:
  • Ciliated columnar cells: are the most abundant type, which have cilia to move mucus towards the nose.
  • Goblet cells: produce mucus, and numerous in some areas.
  • Brush cells: are fewer in number with microvilli and are chemosensory receptors connected to nerve endings.
  • Small granule cells (Kulchitsky cells): part of the diffuse neuroendocrine system (DNES).
  • Basal cells: stem cells that give rise to other epithelial cells (responsible for the pseudostratified appearance.)

Components of the Respiratory System - The Nose and Nasal Cavity

• Specialized organ at the entrance of the respiratory system.
• Consists of external nose and internal nasal cavity.
• External nose has a supporting framework of bone and hyaline cartilage, covered in muscle and skin, lined with mucous membrane.
• Air is filtered, heated and humidified before reaching the lungs.
• Nasal vestibule, initially lined by keratinized stratified squamous epithelium, then transitions to pseudostratified columnar epithelium. Contains hairs for filtering large particles.

Components of the Respiratory System - The Nose and Nasal Cavity (cont'd)

• Nasal cavity is a large space in the skull, inferior to the nasal bone.
• Nasal septum divides the cavity into right and left sides.
• Communicates with the pharynx through internal nares (choanae).
• Nasal conchae (turbinates) increase surface area, covered by respiratory epithelium. This helps with heating and humidifying air.

Additional Respiratory System Components

• Paranasal sinuses: cavities in frontal, maxillary, ethmoid and sphenoid bones of the skull; lined with respiratory epithelium with fewer goblet cells; help in producing mucous.
• Nasal cavities and ducts: include tear ducts (nasolacrimal) and ducts from the paranasal sinuses that open into the nasal cavity. The nasal cavity boundaries extend from the external nares to the pharyngeal opening posteriorly.

Pharynx

• Tube-like structure extending from the internal nares to the larynx.
• Three parts: nasopharynx, oropharynx, and laryngopharynx.
  • Nasopharynx: lined with respiratory epithelium
  • Oropharynx : responsible for air, food, and drink; lined with nonkeratinized stratified squamous epithelium
  • Laryngopharynx: involved in both respiratory and digestive processes. Lined by nonkeratinized stratified squamous epithelium

Larynx

• Connects the laryngopharynx to the trachea.
• Made up of nine pieces of cartilage.
• Contains vocal cords that produce vibrations for speech.
• Lined mostly with pseudostratified columnar epithelium with goblet cells, except the vocal cords, which are lined with a stratified squamous epithelium.

Trachea

• Tubular passageway for air.
• Located anterior to the esophagus.
• Extends from the larynx to the superior border of T5.
• Composed of 16-20 hyaline cartilage rings to prevent collapse.
• Includes a mucosa with pseudostratified columnar epithelium, submucosa with seromucous glands, hyaline cartilage rings and a layer of areolar connective tissue called adventitia.

Bronchi

• Two major branches from the trachea that lead to the left and right lungs.
• Right bronchus is wider, shorter and more vertical than the left bronchus.
• Lined by pseudostratified ciliated columnar epithelium with goblet cells.
• Cartilage rings give structure and help prevent collapse.
• Changes to simple columnar or cuboidal epithelium in smaller bronchioles.

Bronchioles

• Smallest airways in the lungs that continue to branch.
• Do not have cartilage in their walls—giving flexibility.
• Lined by club (Clara) cells and simple cuboidal epithelium. Important for surfactant production and detoxification.
• Respiratory bronchioles connect to the alveolar ducts and sacs.

Alveolar ducts and Alveoli

• Alveoli: sacs that form the main site of gas exchange.
  • Thin walled structures lined with simple squamous epithelium.
• Essential for gas exchange to occur between air and blood, with a high surface area.
• Alveolar walls are covered in a network of capillaries.
• Consist of two types of alveolar cells: type I (flat, squamous) and type II (cuboidal) which produce surfactant.

This quiz covers the structure and function of the respiratory system, detailing the upper and lower divisions as well as the conducting and respiratory zones. Review key concepts and cellular structures involved in the respiratory system as outlined in the histology study notes.