Structure and function of the upper airways

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 is rarely 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. The larynx came up in Question 14 from the second paper of 2020. 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.  Additionally, though the college has never asked about the role of upper airway musculature in respiration (focusing on the diaphragm and chest wall instead), but the possibility remains that they one day will. In order to prepare for this horrifying possibility, this chapter has been compiled mainly using the 8th edition of Last's. For an excessively detailed review of the upper aiway muscles, one may also turn to the 2009 article by Jordan & White, which mainly deals with sleep apnoea.

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

Question 14 from the second paper of 2020 asked specifically for the anatomy of the larynx. A surprising amount of detail was expected. "Candidates were expected to address the location of the larynx, its relations, the cartilages (single and paired), ligaments, muscles (intrinsic and extrinsic), innervation (sensory and muscular) and blood supply (including venous drainage)". As will soon become apparent, that is quite a lot: 

• 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
• Cartilages:
  • Paired: arytenoid, corniculate and cuneiform​​​​​​​
  • Unpaired: epiglottis, thyroid and cricoid​​​​​​​
• Ligaments:
  • ​​​​​​​Intrinsic:  cricothyroid ligament and quadrangular membrane
  • Extrinsic: thyrohyoid membrane, median and lateral thyrohyoid ligament, hyo-epiglottic ligament, cricotracheal ligament.
• Muscles:
  • ​​​​​​​Intrinsic: cricothyroid, thyroarytenoid, posterior cricoarytenoid, lateral cricoarytenoid, transverse and oblique arytenoids. 
  • Extrinsic: supra and infrahyoid group of muscles: digastric, mylohyoid, stylohyoid, omohyoid, thyrohyoid. Also sternohyoid and sternothyroid.
• 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

Role of the upper airway muscles in respiration

The upper airway is full of muscles, the anatomy of which is the nightmarish province of ENT and maxillofacial surgeons. For the intensivist, it will likely suffice to know that their function in respiration is basically to not collapse. And to collapse, apparently, is their natural tendency. As during inspiration the intrathoracic pressure drops by a few fractions of a cm H₂O, so the upper airway pressure also drops, and the squishy mass of tissue which is your upper airway would naturally implode upon itself if it did not make a special effort to stay open. This special effort manifests as a muscular contraction which must by all accounts be viewed as an important part of the respiratory muscle function at least as much as the diaphragm, insofar as its absence would result in a failure of airflow.

This upper airway muscle contraction is a coordinated reflex arc, known as the pharyngeal dilator reflex. As for many reflexes, the arc is activated by mucosal stretch receptors, which respond to negative intrapharyngeal pressure. This was investigated beautifully by Horner et al (1991). The authors demonstrated the role of pharyngeal receptors by recording an EMG pharyngeal dilator muscles (mainly genioglossus) while smearing cocaine paste on the mucosae of healthy volunteers to anaesthetise those receptors ("minor side effects that were noted after the study included ... increased alertness", the authors cackled). The responses to negative pressure were extremely brrisk, averaging 34 millisconds - well faster than any possible voluntary response, confirming that a reflex arc was responsible. Further cocaine help to confirm that the trigeminal, superior laryngeal and the glossopharyngeal nerves were involved in this reflex arc.

Thus, genioglossus is thought to be the muscle most responsible for upper airway patency. It is the largest of the group of airway dilator muscles, which also include several others:

The genioglossus is innervated by the medial branch of the hypoglossal nerve, the unilateral lesion of which will result in the tongue deviating towards the side of the lesion when it is protruded. As one can plainly see from where it is placed anatomically, the contraction of the genioglossus pulls the base of the tongue and the hyoid bone anteriorly, opening the airway (it is in essence the same as the thing you do with the jaw thrust). It does this automatically when you assume a supine position,  because otherwise gravity would pull the base of the tongue down, occluding the airway.

The activation of the pharyngeal dilator reflex is moderated by descending input into the involved cranial nerve nuclei. During states of decreased consciousness (be they sleep or anaesthesia) this airway reflex is depressed, and the airway may collapse (Fregosi & Ludlow, 2014).