Anatomy and Histology of the Respiratory System

The respiratory system is divided into the upper respiratory tract and the lower respiratory tract.


  • Consist of external nose and nasal cavity
  • External nose
    • Visible structure that forms a prominent feature of the face
    • Largest part is composed of hyaline cartilage plates
    • Bridge of the nose consists of the nasal bones plus extension of the frontal and maxillary bone Illu_nose_nasal_cavities
  • Nasal cavity
    • Plays an important role in breathing
    • Extends from the nares (nostrils, external opening) to the choanae (openings into the pharynx)
    • Vestibule, anterior part of nasal cavity is lined with stratified squamous epithelium
    • Nasal septum
      • divides nasal cavity into right and left parts
      • anterior (cartilage), posterior (vomer bone and perpendicular plate of the ethmoid bone)
    • Conchae
      • Three bony ridges that modifies the lateral nasal cavity
      • Beneath them are a passageway called a meatus
      • Within superior and middle meatus are openings from paranasal sinuses while the inferior are openings of a nasolacrimal duct
  • Functions
    • Serves as a passageway for air even when the mouth is full of food
    • The anterior vestibule contains hair which traps dust thus cleans the air
    • Humidifies and warms the air
    • Contains the olfactory epithelium at the superior part for sensation of smell
    • Helps determine voice sound with the help of paranasal sinuses


  • Acts as passageway for both digestive (esophagus) and respiratory (larynx) system 
  • Divided into three regions
    • Nasopharynx respiration
      • Location: posterior to choanae and superior to soft palate
      • Soft palate is an incomplete muscle and connective tissue partition that separates nasopharynx and oropharynx. It prevents swallowed materials from entering the nasopharynx and nasal cavity. It is extended by uvula at the posterior
      • Lined with a mucous membrane containing pseudostratified ciliated columnar epithelium with goblet cells
      • Two eustachian or auditory tubes connect the middle ears with the nasopharynx and allow air pressure in the middle and outer ear to equalise.
      • Pharyngeal tonsil helps defend the body against infection is located at the posterior surface
    • Oropharynx
      • Extends from the soft palate to the epiglottis
      • Joints oral cavity through the fauces that is located near to palatine tonsils and lingual tonsils
      • Lines with moist stratified squamous epithelium and protects against abrasion
    • Laryngopharynx
      • Extends from the tip of the epiglottis to the esophagus and larynx
      • Lined with moist stratified squamous epithelium


  • Located anterior part of the throat and extends from the base of tongue to the trachea .
  • Larynx is connected by membranes and/muscles superiorly to the hyoid bone.
  • It consists of an outer casing of nine cartilages connected to one another by muscle and ligaments.



  • The largest of the cartilage is the unpaired thyroid cartilage or Adam’s apple (refer to the shape of the cartilage)
  • The most inferior cartilage is the unpaired cricoid cartilage( ring-shaped)
  • The third unpaired cartilage is the epiglottis.
  • The paired arytenoid cartilage (ladle-shaped) articulate with the posterior, superior border of the cricoid cartilage.
  • The paired corniculate cartilage ( horn-shaped) attached at the superior tips of the arytenoid cartilage.
  • The paired cunieform cartilage ( wedge-shaped) are contained in a mucous membrane anterior to the corniculate cartilage.
  • the superior ligaments are covered by mucous membrane called vestibular folds or false vocal chords
  • The inferior ligaments are covered by mucous membranes vocal folds or true vocal chords

Four functions of  larynx :-

  1. Thyroid and cricoid cartilage maintain an open passageway for air movements
  2. Prevents swallowed materials from entering the lower repirstory tract and regulates the passage of air into and out of lower respiratory tract.
  3. Vocals folds are the primary source of sound production
  4. the pseudostratified  ciliated columnar epithelium lining the larynx produce mucus and debris into the pharynx.

Tracheobronchial Tree

Français :

  • Beginning with trachea to all respiratory passageways
    • Trachea divides to form two main bronchi, which extends to the lungs.
    • The main bronchi–>lobar bronchi (secondary bronchi)–>segmental bronchi (tertiary bronchi)–>bronchioles–>terminal bronchioles
  • The trachea to the terminal bronchioles is a passageway for air movement
    • The area from the trachea to the terminal bronchioles is ciliated to facilitate the removal of inhaled debris
      • Bronchi – pseudostratified ciliated columnar epithelium
      • Larger bronchioles – ciliated simple columnar epithelium
      • Terminal bronchioles – ciliated simple cuboidal epithelium
    • Cartilage helps hold the tube system open (from the trachea to the bronchioles)
      • Main bronchi – C-shaped cartilage connected by smooth muscle
      • Lobar bronchi – smooth muscle forms a layer between the cartilage plates and mucous membrane
      • Terminal bronchioles – no cartilage and prominent smooth muscle
    • Smooth muscle controls the diameter of the tubes (terminal bronchioles) and thereby changes the volume of air moving through them
  • Terminal bronchioles divide to form respiratory bronchioles,
    • It gives rise to alveolar ducts
    • Air-filled chambers called alveoli open into the respiratory bronchioles and alveolar ducts.
    • The alveolar ducts end as alveolar sacs, which are chambers that connect to two or more alveoli
  • The tissue surrounding the alveoli contains elastic fibers, which allows the alveoli to expand during inspiration and recoil during expiration
  • Although respiratory bronchioles and alveoli is not ciliated, debris from the air can be removed by macrophages, then it either move into nearby lymphatic vessels or entrapped in mucus that is swept to the pharynx resp_s10
    • Respiratory bronchioles
      • Walls consist of collagenous and elastic connective tissue with bundles of smooth muscle
      • Lines with simple cuboidal epithelium
    • Alveolar ducts and alveoli
      • Lines with simple squamous epithelium
  • Two types of cells from alveolar wall
    • Type I pneumocytes
      • Thin squamous epithelial cells that form 90% of alveolar space
      • most of gas exchange takes place here
    • Type II pneumocytes
      • round or cube-shaped secretory cells that produces surfactant, which makes easier for alveoli to expand during inspiration



  • Principal organs of respiration
  • Conical shape, base resting on the diaphragm, apex superior to clavicle
  • Thoracic cavity contains two lungs
  • Hilum is a region on the medial surface of the lungwhere main bronchus, blood vessels, nerves and lymphatic vessels, enters or exits the lungs
  • The lungs are divided into lobes, bronchopulmonary segments, and lobules



  • Lobes
    • Separated by deep, prominent fissures on the surface of the lungs
    • Each lobe is supplied by a lobar bronchus
    • Subdivided into bronchopulmonary segments
  • Bronchopulmonary segments
    • Separated from each other by connective tissue partitions
    • Supplied by segmental bronchi
    • Subdivided into lobules
  • Lobules
    • Separated by incomplete connective tissue walls
    • Supplied by the bronchioles

Thoracic Wall and Muscles of Respiration

  • Thoracic wall consists of vertebrae, ribs, the sternum and muscles that allow expansion of the thoracic cavity
  • Contraction of the diaphragm increases thoracic volume
  • Muscles can elevate the ribs and increase thoracic volume or depress the ribs and decrease thoracic volume



  • Pleural serous membrane form pleural cavity that surrounds the lungs and protect against friction
    • Parietal pleura covers the inner thoracic wall, superior surface of diaphragm and mediastinum
    • Visceral pleura covers the surface of the lungs at the hilum
  • Mediastinum separates the pleural cavities
    • Pleura cavity filled with pleura fluid, produced by pleura membranes
      • Acts as lubricant, allowing the parietal and visceral pleura membranes to slide each other during ventilation
      • Helps hold the two membranes together

Blood Supply

  • Deoxygenated blood is transported to the lungs through the pulmonary arteries, while oxygenated blood leaves through the pulmonary veins
  • Oxygenated blood is mixed with a small amount of deoxygenated blood from the bronchi

Lymphatic Supply

  • The superficial and deep lymphatic vessels drain lymph from the lungs
    • Superficial lymphatic vessels are deep to visceral pleura – drain lymph from superficial lung tissue and visceral pleura
    • Deep lymphatic vessels follow the bronchi – drain lymph from bronchi and associated connective tissues
  • Both lymphatic vessels exit the lung at the hilum
  • No lymphatic vessels are located in the walls of alveoli

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