Upper Airways

Pharynx

Pharyngeal constrictors (inferior, middle and superior) allow for swalloing
Pharyngeal dilators contract to maintain patency of the pharynx so that air can flow to the lungs
During inspiration negative pressure may cause the pharynx to collapse if it were not for the pharyngeal dilators:
- Genioglossus which causes the tongue to protrude forward and away from the pharyngeal wall
- Palatal muscles control the stiffness and position of the palate, tongue and pharynx as well as the shape of the uvula
- Muscles influencing the position of the hyoid (e.g. geniohyoid) exhibit phasic activity (i.e. their activity is increased during inspiration)
- While asleep (espeically during stage 3 non-rapid eye movement sleep) there is a loss in pharyngeal muscle tone which can precipitate snoring and obstructive sleep apnoea
- Apart from ketamine, all the other anaesthetic and sedative agents reduce central respiratory drive and pharyngeal muscle tone

During exhalation, the vocal cords are adducted slightly to create resistance to flow ⇒ positive end expiratory pressure of 3-4cm $H X_{2} O$ (physiological PEEP) ⇒ prevents alveolar collapse and maintains functional residual capacity
- During intubation, patients can no longer adduct their vocal cords and can’t maintain physiological PEEP ⇒ atelactasis and $\dot{V} / \dot{Q}$ mismatch; it is therefore common to apply extrinsic PEEP ( $PEEP_{e}$ ) at physiologic levels (3-5 cm $H X_{2} O$ ) to maintain FRC and prevent atelactasis following intubation
  - PEEP can increase intrathoracic pressure ⇒ ↓ venous return ↑ venous pressure which may not be favourable in raised ICP or tonsillectomy (increase bleeding at tonsillar bed can obstruct surgeon’s view)

Trachea starts at lower border of cricoid cartilage at the level of C6 and bifurcates at the carina (T4/5 level)
The right main bronchus is shorter, wider and more vertical than the left (therefore inhaled foreign bodies and ETTs are more likely to enter the right main bronchus)
The right upper lobe bronchus originates from the right main bronchus only 2 cm distal to the carina. In contrast, the left main bronchus bifurcates 5 cm from the carina
- Hence left-sided double lumen ETTs are favoured over right sided tubes for one lung ventilation
Bronchioles
- First generation that does not contain cartilage
- Have a layer of smooth muscle that governs bronchodilation and constriction
- $β_{2}$ agonists and anticholinergics cause bronchodilation
Alveoli
- Consist of type 1 pneumocytes (allow for gas exchange, thin cells), type 2 pneumocytes (specialised secretory cells that coat the alveoli with pulmonary surfactant) and alveolar macrophages

Immunological and lung defence
Metabolic and endocrine
- Inactivation of noradrenaline, serotonin, prostaglandins, bradykinin and acetylcholine
Synthesis of surfactant, nitric oxide and heparins
Synthesis, storage and release of pro-inflammatory mediators including histamine, eicasonoids, endothelin, platelet aggregating factor and adenosine
Some anaesthetic drugs undergo first pass metabolisation in the lungs (e.g. noradrenaline, lignocaine, fentanyl)