Respiratory System Embryology and Structure

Embryological Development

  • Endoderm:
    • Outpocketing of the gut tube.
    • Forms the lower respiratory tract and part of the pharynx.
  • Ectoderm:
    • Involves the oral and nasal cavities.
    • Invaginations meet the endodermal portion in the pharyngeal region.
  • Mesoderm:
    • Involved in smooth and skeletal muscle.
    • Connective tissue of bronchial tubes and related structures.
    • Pleural membranes (serous membrane made of mesodermal epithelial tissue).

Fundamental Functions

  • Gas Exchange:
    • Exchange of oxygen and carbon dioxide.
  • Vocalization:
    • Making sounds for communication.
    • Phonation: actual part of vocalization involving making words.
  • Protection:
    • Protecting the body from microorganisms.

Structural Classification

  • All structures are lined with a mucous membrane (epithelial tissue membrane).

Upper Respiratory Tract (URT)

  • Structures:
    • Oral cavity
    • Nasal cavities
    • Nasopharynx
    • Larynx (sometimes classified as part of the lower respiratory tract in older classifications)

Lower Respiratory Tract (LRT)

  • Structures:
    • Trachea (windpipe)
    • Bronchi
    • Bronchioles

Functional Divisions

  • Conducting Zone:
    • Air is transported in and out of the system.
    • Includes:
      • Cavities: oral cavity, nasal cavities, paranasal sinuses (four pairs named for the bones they are in)
      • Tubes: larynx, trachea, bronchi, bronchioles, terminal bronchioles
  • Respiratory Zone:
    • Gases (oxygen and carbon dioxide) are exchanged.
    • Analogous to the capillary end of blood vessels where exchange occurs.
    • Occurs within the lungs.
    • Includes:
      • Respiratory bronchioles
      • Alveolar ducts
      • Alveolar sacs
      • Alveoli

Upper Respiratory Tract Details

Nasal Cavities

  • Functions:
    • Warming and moistening air (humidifying).
    • Filtering out particulate matter (large and small).
  • Structures:
    • Two cavities separated by a bony and cartilaginous septum (wall).
    • Conchae (turbinates): increase surface area for warming, moistening, and filtering.
    • Lined with mucous membrane (containing lots of water).
    • Olfactory nerve (cranial nerve number one):
      • Located in the roof of the cavities, running along the cribriform plate.
      • Involved in:
        • Olfaction (sense of smell)
        • Gustation (sense of taste) - most taste occurs through smell.

Paranasal Sinuses

  • Four pairs of hollow spaces within bones:
    • Frontal
    • Maxillary
    • Ethmoid
    • Sphenoid
  • Functions:
    • Increase surface area for warming, moistening, and filtering air.
    • Lighten the skull.
    • Involved in resonation of voice.

Oral Cavity

  • To be discussed in detail with the digestive system.

Pharynx

  • Region shared by both the respiratory and digestive systems.
  • Three distinctive parts:
    • Nasopharynx:
      • Posterior to the nasal cavity.
      • Contains the internal acoustic meatus.
      • Contains adenoids (pharyngeal tonsils).
    • Oropharynx: Posterior to the oral cavity.
    • Hypopharynx (Laryngopharynx):
      • Posterior to the larynx.

Tonsils

  • Masses of lymphatic tissue (not well-organized, encapsulated).
  • Form a ring around the upper respiratory system.
  • Three types:
    • Pharyngeal Tonsils:
      • Also known as adenoids (when swollen).
      • Near the auditory tube opening.
    • Palatine Tonsils:
      • At the back of the soft palate.
      • Commonly swell during tonsillitis or strep infections.
    • Lingual Tonsils:
      • At the base of the tongue.

Larynx

  • Functions as a passageway for air and contains the vocal folds.
  • Location:
    • Anterior border: thyroid cartilage (Adam's apple in males, due to testosterone); hyaline cartilage.
    • Superior border: epiglottis (elastic cartilage).
    • Inferior border: cricoid cartilage (hyaline cartilage).
  • Epiglottis:
    *Forms a flap to cover the laryngeal opening during swallowing, protecting the airway.
  • Vocal Folds (Vocal Cords):
    • Comprised of elastic connective tissue.
    • Folds within the thyroid cartilage.
    • Stretched by skeletal muscle.
    • Passing air through creates sound.
    • Sound is shaped into words using lips, teeth, tongue, cheeks, etc.
    • Resonation of voice occurs here.

Lower Respiratory Tract

Trachea (Windpipe)

  • Composed of a fibroelastic cartilaginous tube.
  • Approximately 4.5 inches long and 1 inch in diameter.
  • Extends from the cricoid cartilage to the primary bronchi.
  • Composed of a series of C-shaped cartilaginous bands held together by fibrous connective tissue.
  • Cross-sectional anatomy:
    • Located anterior to the esophagus and the body of the vertebra.
    • Trachealis muscle:
      • Muscle that connects the ends of the C-shaped cartilages posteriorly.
      • Contracts to pull the C-shaped ends inward, allowing the esophagus to expand during swallowing.

Bronchi

  • Bronchus (singular).
  • Two primary bronchi (right and left).
  • Penetrate the root (hilus/hilum) of the lung.
  • Divide into:
    Primary→Secondary→TertiaryPrimary \rightarrow Secondary \rightarrow Tertiary bronchi within the lung.
  • Made of cartilage (hyaline).
  • Bronchioles:
    * Smaller tubes formed beyond the tertiary bronchi.
    * No hyaline cartilage; made of smooth muscle and loose and fibrous connective tissue.
    * Lined with a mucous membrane.
    * Divide approximately 15-17 times.