Tracheobronchial Tree

• The trachea, bronchi and bronchioles from the tracheobronchial tree – a system of airways that allow passage of air into the lungs, where gas exchange occurs. These airways are located in the neck and thorax.
• The trachea marks the beginning of the tracheobronchial tree. It arises at the lower border of cricoid cartilage in the neck, as a continuation of the larynx. It travels inferiorly into the superior mediastinum, bifurcating at the level of the sternal angle (forming the right and left main bronchi). As it descends, the trachea is located anteriorly to the oesophagus, and inclines slightly to the right.
• The trachea is held open by cartilage in C-shaped rings. The free ends of these rings are supported by the trachealis muscle.
• The trachea and bronchi are lined by ciliated pseudostratified columnar epithelium, interspersed by goblet cells, which produce mucus. The combination of sweeping movements by the cilia and mucus from the goblet cells forms the functional mucociliary escalator. This acts to trap inhaled particles and pathogens, moving them up out of the airways to be swallowed and destroyed.
• At the bifurcation of the primary bronchi, a ridge of cartilage called the carina runs anteroposteriorly between the openings of the two bronchi. This is the most sensitive area of the trachea for triggering the cough reflex, and can be seen on bronchoscopy.
• The trachea receives sensory innervation from the recurrent laryngeal nerve. Arterial supply comes from the tracheal branches of the inferior thyroid artery, while venous drainage is via the brachiocephalic, azygos and accessory hemiazygos veins.
• At the level of the sternal angle, the trachea bifurcates into the right and left main bronchi. They undergo further branching to produce the secondary bronchi. Each secondary bronchi supplies a lobe of the lung, and gives rise to several segmental bronchi.

  Along with branches of the pulmonary artery and veins, the main bronchi make up the roots of the lungs.
• Right main bronchus – wider, shorter and descends more vertically than its left-sided counterpart. Clinically, this results in a higher incidence of foreign body inhalation. The right superior lobar bronchus arises before the right main bronchus enters the hilum.
• Left main bronchus – passes inferiorly to the arch of the aorta, and anteriorly to the thoracic aorta and oesophagus in order to reach the hilum of the left lung.
• Within the lungs, the main (primary) bronchi branch into lobar (secondary) bronchi. Each secondary bronchi supplies a lobe of the lung, thus there are 3 right lobar bronchi and 2 left. The lobar bronchi then bifurcate into several segmental (tertiary) bronchi, each of which supplies a bronchopulmonary segment. Bronchopulmonary segments are subdivisions of the lung lobes, and act as the functional unit of the lungs.
• The structure of bronchi are very similar to that of the trachea, though differences are seen in the shape of their cartilage. In the main bronchi, cartilage rings completely encircle the lumen. However in the smaller lobar and segmental bronchi cartilage is found only in crescent shapes.
• The bronchi derive innervation from pulmonary branches of the vagus nerve (CN X). Blood supply to the bronchi is from branches of the bronchial arteries, while venous drainage is into the bronchial veins.
• The segmental bronchi undergo further branching to form numerous smaller airways – the bronchioles.
• The smallest airways, bronchioles do not contain any cartilage or mucus-secreting goblet cells. Instead, club cells produce a surfactant lipoprotein which is instrumental in preventing the walls of the small airways sticking together during expiration.
• Initially there are many generations of conducting bronchioles, which transport air but lack glands are not involved in gas exchange. Conducting bronchioles then eventually end as terminal bronchioles. These terminal bronchioles branch even further into respiratory bronchioles, which are distinguishable by the presence of alveoli extending from their lumens.
• Alveoli are tiny air-filled pockets with thin walls (simple squamous epithelium) and are the sites of gaseous exchange in the lungs. Altogether there are around 300 million alveoli in adult lungs, providing a large surface area for adequate gas exchange.
• Asthma is a chronic inflammatory disorder of the airways, characterised by hypersensitivity, reversible outflow obstruction and bronchospasm.

  There is remodelling of the small airways, causing increased smooth muscle thickness around the bronchioles, damaged epithelium an a thickened basement membrane.
• ‘Asthma attacks’ are acute exacerbations of the condition whereby a trigger (e.g. allergens, exercise) causes sudden inflammation and contraction of the smooth muscle around bronchioles (bronchospasm). This narrows the airways, causing difficulty in breathing and wheezing, a characteristic feature of asthma.
• Trachea

  • arises at the lower border of the cricoid cartilage
  • extends inferiorly into the superior mediastinum
  • continuation of the larynx
  • bifurcation at the level of the angle of Louis (manubriosternal joint)
  • Situated anterior to the oesophagus
• Bronchi

  • bifurcation of trachea forms right and left bronchi
  • branching continues, creating secondary bronchi
  • secondary bronchi each supply a lobe of the lung
  • Segmental bronchi arise from branching of secondary bronchi
• Bronchioles - arise from further branching of segmental bronchi
• Lungs

  • Located within the thoracic cavity
  • One lung on each side of the mediastinum
  • Pleural cavity surrounding each lung, formed by visceral and parietal pleura
  • Left lung has 2 lobes, right lung has 3 lobes
• The tracheobronchial tree = trachea + bronchi + bronchioles
• • Patency of airway is maintained by cartilaginous rings (C-shaped)
  • Lined by ciliated pseudostratified columnar epithelium. Goblet cells present at intervals throughout., forming the “mucociliary escalator”
  • When the trachea bifurcates, the carina runs anteroposteriorly between the two bronchi. This is the point at which the cough reflex is most likely to be triggered within the trachea
• The trachea bifurcates at the level of the 4th/5th thoracic vertebrae (manubrio-sternal angle)
• Bronchi

  • Right main bronchus: wider, shorter, more vertical
  • Left main bronchus: narrower, longer, more horizontal
  • Secondary bronchi each supply a lobe of the lung: 3 right lobar bronchi, 2 left lobar bronchi
  • Segmental bronchi each supply a bronchopulmonary segment
  • Cartilaginous rings within the main bronchi are circular in shape, surrounding the entire lumen of the airway.
  • Within the secondary and segmental bronchi, cartilage is crescent shaped
• Listen to chest when inserting endotracheal tube to check there is air entry on both sides and it has not entered the right main bronchus.
• Bronchioles

  • No cartilage or goblet cells present
  • Surfactant lipoprotein produced by club cells
  • Conducting bronchioles are not involved in gas exchange. They end in terminal bronchioles.
  • Terminal bronchioles branch to form respiratory bronchioles
  • Alveoli (sites of gaseous exchange) extend from respiratory bronchioles.
• • Smooth muscle is present from the trachea to the terminal bronchioles
  • It contains beta-adrenergic and muscarinic receptors
  • Beta-agonists and muscarinic antagonists relax the muscles, this increases the radius of the airways, which reduces the resistance to ventilation
• Laplace’s Law

  • Alveoli are covered in water
  • Water molecules are attracted to each other – surface tension
  • Surface tension exerts a collapsing force on the alveoli
  • Laplace’s law states that collapsing pressure = surface tension/radius
  • Therefore smaller radius = stronger collapsing force
  • This is a positive feedback loop – BAD NEWS
• • Surfactant reduces surface tension
  • The quantity of surfactant is relatively constant in each alveoli
  • Therefore as the radius gets smaller the concentration of surfactant gets larger
  • Therefore as the radius gets smaller the surface tension (and therefore collapsing pressure) gets lower
  • This is a negative feedback loop – GOOD NEWS
• The right main bronchus is more vertical than the left therefore the right lower lobe is the most likely place to get an aspiration pneumonia.
• Giving corticosteroids to pregnant woman in premature labour can encourage baby to produce surfactant earlier/more which can reduce the need for ventilation after birth. Surfactant production does not usually occur until 34 weeks.
• Larynx structure

  • Anterior triangle of the neck
  • Level C3 – C6
  • Suspended from the hyoid bone
  • Cartilaginous skeleton held together by ligaments and membranes
  • Three divisions:

  1. Supraglottis
  2. Glottis
  3. Subglottis

  • Majority lined by ciliated pseudostratified columnar epithelium
  • True vocal cords lined by stratified squamous epithelium
• Arterial Supply Larynx

  • Superior laryngeal artery - Branch of superior thyroid artery
  • Inferior laryngeal artery - Branch of inferior thyroid artery
• Venous drainage larynx

  • Superior laryngeal vein - Drains to IJV via superior thyroid vein
  • Inferior laryngeal vein - Drains to left brachiocephalic vein via inferior thyroid vein
• Innervation of larynx

  Recurrent laryngeal nerve

  • Sensory innervation of infraglottis
  • Motor innervation to ALL internal laryngeal muscles EXCEPT cricothyroid

  Superior laryngeal nerve

  • Sensory innervation of supraglottis
  • Motor innervation of cricothyroid via external branch
• Lymph drainage of larynx

  Supraglottic lymph nodes

  • Drain into the superior deep cervical nodes and the pre-epiglottic nodes

  Subglottic lymph nodes

  • Drains into inferior deep cervical nodes and the pre-laryngeal nodes
• Right recurrent laryngeal nerve at higher risk of damage during surgery than left nerve
• suprahyoid muscles

  • stylohyoid
  • digastric
  • mylohyoid
  • geniohyoid

  Together with stylopharnygeus, elevate Larynx