This chapter is most relevant to Section F10(iv) from the 2017 CICM Primary Syllabus, which expects the exam candidates to "Describe the function and structure of the upper, lower airway and alveolus". The alveolus and lower airways are sufficiently interesting that they should get their own chapters. Here, we will take a trip down the air passages from the vibrissae to the larynx.

Unfortunately, like most anatomy, this is an exercise in listing facts, which is susceptible to being mindlessly memorised and regurgitated. In terms of exam value, this topic has never been explored in the written exam. Question 24 from the second paper of 2016 asked about the structure of the airways, but only the adult human trachea and bronchi were asked about. Surgical anatomy of the upper airway has come indirectly in various ways, eg. in the context of discussing anatomy for intubation or tracheostomy, but no direct "describe the anatomical relations of the larynx" questions have ever been asked. In order to prepare for this horrifying possibility, this chapter has been compiled mainly using the 8th edition of Last's. 

The distinction between upper and lower airways

When one refers to the upper airways, one generally means "everything above the larynx".  This is a nod to the gatekeeper function of the larynx as the separator of the "shared" aerodigestive functions of the pharynx and mouth from the purely respiratory functions of the lower airway. Gaga et al (2000) and most other authors group the larynx together with the upper structures. 

External nose

  • Landmarks: nares, septum, bridge.
  • Basic structural anatomy: Cartilaginous structure, supported by hyaline cartialge and nasal bones. Beyond the vestibule of the nostrils, some anterior skin transitions into a respiratory mucous epithelium at the mucocutaneous junction
  • Relations: Bony nasal aperture (piriform aperture) is bounded superiorly by nasal bones and laterally / inferiorly by the maxillae. The external nose surrounds and is anatomically separated from the nasal cavity by the exterior nares.
  • Blood supply: ​​​​​extensive arterial supply with multiple areas of anastomosis
    • dorsal nasal artery (a terminal branch of the ophthalmic)
    • external nasal artery (from the anterior ethmoidal)
    • lateral nasal and septal branches of the facial artery
    • superior labial branch of the facial artery
  • Venous drainage:
    • Vens are collateral with the arteries, and drain into the pterygoid plexus, facial vein or cavernous sinus. Drainage into the saggital sinus is a normal anatomical variation. 
  • Innervation: 
    • external nasal nerve (the terminal part of the anterior ethmoidal)
    • supratrochlear and infratrochlear nerves
    • nasociliary nerve
    • nasal branches of the infraorbital nerve
  • Function:
    • Filtration of inspired air (separation of inhaled particles)

Nasal cavity

  • Landmarks: nares, conchae
  • Basic structural anatomy: three areas:
    • Vestibular area, lined by skin (extends 1cm into the cavity)
    • Respiratory area, lined by respiratory mucous membrane with pseudostratified ciliated columnar epithelium. Contains the three concha which project into the cavity.
    • Olfactory area, at the roof of the cavity (lower boundary is the superior conchae)
  • Relations: 
    • Floor of the nose is the roof of the mouth
    • Roof is the narow junction of the lateral walls
    • Lateral wall superiorly is the medial wall of the orbit
    • Lateral wall inferiorly is the medial wall of the maxillary sinus
    • Medial wall is the septum
  • Blood supply: ​​​​​extensive arterial supply: 
    • sphenopalatine artery (terminal branch of the maxillary artery)
    • septal branch of the superior labial artery
    • ascending branch of the greater palatine artery
    • All of these come together into an anastomosis in the lower anterior septum (Little's area), which is called Kieselbach's Plexus (where epistaxis commonly occurs)
  • Venous drainage:
    • Veins are collateral with the arteries,
    • Drainage is in multiple directions: pterygoid plexus via the sphenopalatine foramen, to the facial vein, to the ophthalmic vein and to the inferior cerebral vein.
  • Lymphatic drainage:
    • To submandibular, deep cervical and retropharyngeal nodes
  • Innervation: 
    • Vestibular area: infraorbital nerve
    • Olfactory area: olfactory nerve
    • Respiratory area: lateral wall supplied by six nerves, septum by four nerves:
      • anterior ethmoidal nerve
      • anterior superior alveolar nerve
      • lateral posterior superior nasal branches from the pterygopalatine ganglion
      • posterior inferior nasal branches of the greater palatine nerve
      • olfactory nerve
      • medial posterior superior nasal nerve
      • nasopalatine nerve
  • Function:
    • Humidification and warming of inspired air
    • Reclamation of expired moisture and heat
    • Olfaction and sense information about air temperature
    • Speech (nasalisation)
    • Sneezing (protective reflex)

Oral cavity

  • Landmarks: lips, cheeks, dentition
  • Basic structural anatomy: two areas:
    • Vestibule: space between the teeth and lips/cheeks
    • Oral cavity: space beyond the teeth
  • Relations: 
    • Floor is the tongue
    • Roof is the hard and soft palate
    • Lateral walls are the teeth and cheeks
    • Bounded by the oral vestibule and lips anteriorly
    • Posteriorly, separated from the pharynx by the anterior tonsillar pillars
  • Blood supply: ​​​​​ 
    • greater palatine artery
    • Tongue: lingual artery
  • Venous drainage:
    • Veins are collateral with the arteries,
    • Drainage is to the pterygoid plexus and the pharyngeal plexus
  • Lymphatic drainage:
    • To deep cervical and retropharyngeal nodes
  • Innervation: 
    • Oral cavity: Greater palatine nerve (from the maxillary via the pterygopalatine ganglion)
    • The anterior part of the palate:  two nasopalatine nerves
    • Tongue sensory: 
      • Anterior 2/3: lingual nerve & chorda tympani
      • Posterior 1/3: Glossopharyngeal nerve
    • Tongue motor: Hypoglossal, nerve, except for the palatoglossus muscle (supplied by cranial part of the vagus)
  • Function:
    • Respiration (alternative airway)
    • Mastication
    • Saliva (digestive and immunological roles)
    • Speech
    • Sensory roles, including taste

Pharynx

  • Basic structural anatomy: 
    •  Fibromuscular tube, ~ 12cm in length
    • Walls have 4 layers: 
      • mucous membrane
      • submucous layer (or fibrous layer)
      • muscular layer
      • buccopharyngeal fascia
    • Can be divided into the nasopharynx, oropharynx and laryngopharynx
  • Relations: 
    • Anteriorly: communicates with nose, mouth and larynx. 
    • Inferiorly: communicates with the oesophagus (at the level of C6)
    • Posteriorly: slides freely along the prevertebral fascia (separated by the retropharyngeal potential space)
  • Blood supply: ​​​​​
    •  Multiple arteris: ascending pharyngeal, ascending palatine, lingual, tonsillar, greater palatine, the artery of the pterygoid canal, and the superior and inferior laryngeal arteries.
  • Venous drainage:
    • To the pharyngeal venous plexus
    • To the pterygoid plexus
    • Directly into the internal jugular vein
    • To the inferior thyroid veins.
  • Lymphatic drainage:
    • To deep cervical and retropharyngeal nodes
  • Innervation: 
    •  Motor: all the muscles are supplied by the pharyngeal plexus except for stylopharyngeus, (supplied by the glossopharyngeal nerve)
      • Pharyngeal plexus is combined from the vagus, glossopharyngeal nerves and the cervical symphathetic chain
    • Sensory: maxillary, glossopharyngeal and the internal and recurrent laryngeal nerves
  • Function
    • Swallowing
    • Separation of the digestive and respiratory tract
    • Phonation
    • Immune function (Waldeyer's ring)

Larynx

  • Landmarks:
    • Laryngeal prominence ("Adam's apple")
  • Basic structural anatomy: 
    • Considered part of the respiratory tract
    • Lined by pseudostratified columnar ciliated epithelium
    • Divides the airways into "upper" and "lower"
  • Relations: 
    • Superiorly: bounded by the hyoid bone
    • Anteriorly, covered by skin and protected by the thyroid cartilage
    • Inferiorly: becomes continuous with the trachea at the level of C6
    • Posteriorly: projects into the laryngopharynx
    • Laryngeal inlet:
      • Faces backwards and upwards
      • Bounded anteriorly by the upper edge of the epiglottis,
      • Bounded laterally and postriorly by the aryepiglottic folds
      • Bounded posteriorly by the interarytenoid fissure
    • Divided by the vocal folds into upper and lower half
  • Blood supply: ​​​​​
    • Upper half: superior laryngeal branch of the superior thyroid artery
    • Lower half: inferior laryngeal branch of the inferior thyroid artery
  • Venous drainage:
    • Upper half: superior laryngeal veins which empty into the superior thyroid veins
    • Lower half:inferior laryngeal veins to the inferior thyroid veins, which drain into the brachiocephalic veins
  • Lymphatic drainage:
    • upper and lower groups of deep cervical nodes
  • Innervation: 
    • All the muscles of the larynx are supplied by the recurrent laryngeal nerve except cricothyroid which is innervated on its external surface by the external laryngeal nerve.
  • Function
    • Respiration (conductive airway)
    • Swallowing
    • Phonation
    • Cough reflex

References

Mete, Aslı, and İlknur Hatice Akbudak. "Functional Anatomy and Physiology of Airway." Tracheal Intubation. IntechOpen, 2018.

Hyde, Dallas M., Qutayba Hamid, and Charles G. Irvin. "Anatomy, pathology, and physiology of the tracheobronchial tree: emphasis on the distal airways." Journal of Allergy and Clinical Immunology 124.6 (2009): S72-S77.

Neil, J. Hardie, Walter Gilmour, and F. J. Gwynne. "Anatomy of bronchial tree." British medical journal 1.4079 (1939): 495.

Marchant, Warwick. "Anatomy of the larynx, trachea and bronchi." Anaesthesia & intensive care medicine 6.8 (2005): 253-255.

Knight, Darryl A., and Stephen T. Holgate. "The airway epithelium: structural and functional properties in health and disease." Respirology 8.4 (2003): 432-446.

Sauret, V., et al. "Study of the three‐dimensional geometry of the central conducting airways in man using computed tomographic (CT) images." Journal of anatomy 200.2 (2002): 123-134.

Macklem, Peter T. "The physiology of small airways." American journal of respiratory and critical care medicine157.5 (1998): S181-S183.

Evans, David J., and Malcolm Green. "Small airways: a time to revisit?." (1998): 629-630.

Weibel, Ewald R., André Frédérick Cournand, and Dickinson W. Richards. Morphometry of the human lung. Vol. 1. Berlin: Springer, 1963.

Mitzner, Wayne. "Mechanics of the lung in the 20th century." Comprehensive Physiology 1.4 (2011): 2009-2027.

Gaga, M., A. M. Vignola, and P. Chanez. "Upper and lower airways: similarities and differences.European respiratory monograph 6 (2001): 1-15.