Histology of the respiratory tract DR SIVA

Learning outcomes At the end of the lecture, the student should be able to Explain the histological features of different parts of respiratory tract. Describe the microscopic anatomy of trachea, bronchi and lungs with help of suitable diagrams. Outline the terminology for the classification of airways, pathways for the blood supply and lymphatic drainage, and the sites of lymphoid tissue. Explain the structure of alveoli and the types of cells in it. Compare the structure of different parts of the respiratory tract. Explain the barrier to the gaseous exchange and capillaries in the walls of the alveoli. Distinguish between different types of airways.

General features of respiratory system Components & basic functions of the respiratory system. Three major parts Ventilating mechanism Conducting portion Respiratory portion

Conducting portions It includes - nasal cavity, nasopharynx, larynx, trachea, bronchi, bronchioles & terminal bronchioles.

Respiratory portion Is distinguished by alveoli, Alveoli are small, saccular structures whose thin walls enable gas exchange between air & blood. These clusters extend from the walls of respiratory bronchioles, alveolar ducts,atria & alveolar sacs.

Overview of respiratory tract histology The basic organization of lung is that of a gland In which vast branching tree of tubes with lined epithelium are in continuity with outside body surface In the beginning of lung development the cells are cuboidal They become simple squamous cells But some cells retain cuboidal epithelium and secrete surfactant

Overview of respiratory tract histology Glands have branching ducts and at the end they have secretory functional unit The respiratory system also have branching such as Trachea Bronchi Bronchioles At the end in the lungs there are alveoli

Conducting passage of respiratory tract Includes : Nasal cavity Pharynx Larynx Trachea Bronchi Bronchioles All these are lined by pseudo stratified columnar epithelium Which is ciliated and have mucous secreting goblet cells Dust and bacteria stick to mucous and swept away by the cilia

Conducting system From nasal cavity to bronchi the epithelium is pseudo stratified columnar epithelium In the bronchioles the epithelium is cuboidal In the alveoli the epithelium is simple squamous epithelium

Conducting passage of respiratory tract The conducting system requires wide and open passages For this support is provided by cartilage

Respiratory or exchange part of respiratory tract It contains millions of small sacs or alveoli These are lined with simple squamous epithelium This epithelium facilitates exchange It also consists of cuboidal surfactant producing cells Surfactant overcomes the tendency of the alveolar walls from collapsing The alveoli are surrounded by capillaries which help in gas exchange Which is the function of life

Pseudo stratified columnar epithelium The main cell of pseudo stratified columnar epithelium is the columnar ciliated cell They sweep mucus toward pharynx Scattered among them are occasional goblet cells They secrete mucous which moistens the epithelial surface The basal are the cells giving rise to ciliated columnar and goblet cells E.g. is the trachea

Pseudo stratified columnar epithelium The pseudo stratified epithelium begins in the nasal cavity and Continues into : Nasopharynx Trachea Bronchi Bronchioles The mucosa provides for moisture for humidification, mucus and vascular plexus for heat exchange

Trachea The trachea is the trunk of the branching respiratory tree Its histological specialization includes skeletal reinforcement in the form of ‘C’ shaped cartilage With respiratory epithelium

Bronchi and bronchioles The trachea divides into bronchi Which continue to divide as bronchioles The cartilage and the thickness of epithelium decreases as the alveoli are reached The bronchi have more cartilage and columnar epithelium While bronchioles lack cartilage and have cuboidal epithelium Mucus glands are located all along the respiratory tree The respiratory tract also have smooth muscle which regulate air passage

Lung The lung consists of functional respiratory units called as alveoli Lung is like sponge Each alveolus shares its wall with adjacent alveolus Alveoli are separated by thin alveolar walls Each alveolus is lined by simple squamous epithelium It allows gaseous exchange In between the two epithelial surfaces lies the capillaries, with RBC and hemoglobin absorbs oxygen and transports to the rest of the body

Lung The interalveolar septum consist of simple squamous epithelium on either side With capillaries sandwiched between them.

Lung Blood air barrier consists of : Plasma membrane of a squamous epithelial cell Thin epithelial cell cytoplasm Plasma membrane on the other side of the epithelial cell Basement membrane which separates the epithelium from the underlying tissue Basement membrane of capillary Plasma membrane of a endothelial cells Cytoplasm of endothelial cell Plasma membrane on the -lumen side of the endothelial cell Blood plasma Finally the membrane of a red blood cell.

Lung The squamous epithelial cells are called Type I pneumocytes Type I cells facilitate gas exchange Scattered among these cells there are occasional large cuboidal cells called Type II pneumocytes These cells secrete surfactant This prevents the sticking together of alveolar walls

Lung The terminology of : Terminal bronchiole opening into gas exchange are called as respiratory bronchiole Respiratory ducts Respiratory sacs They contain smooth muscle for control of air distribution Open into alveoli To keep the alveoli clean there are dust cells which are in fact macrophages

Trachea epithelium and connective tissue

Respiratory epithelium

Trachea

Lung

Lung – respiratory bronchiole and alveoli

Lung, respiratory duct and alveoli

Lung alveoli

Lung with bronchus and artery

End the end of lecture Student should be able to: Describe histological features of different parts of the respiratory tract Microscopic anatomy of trachea and lungs Terminology of airways Blood supply and lymphatic drainage of respiratory tract Structure of alveoli and types of cells in it Compare structure of different part of reparatory tract Explain blood –gas barrier