Abstract
The lower airways can be divided into the larynx and tracheobronchial tree, which is subdivided into the conducting and respiratory zones.
What are the functions of the lung?
The lung has both respiratory and non-respiratory functions:
Respiratory functions are those that facilitate gas exchange:
– Movement of gases between the atmosphere and the alveoli;
– Passage of O2 from the alveoli to the pulmonary capillaries;
– Passage of CO2 from the pulmonary capillaries to the alveoli;
– Synthesis of surfactant.
Non-respiratory functions are:
– Acid–base balance;
– Immunological and lung defence;
– Vascular;
– Metabolic and endocrine.
Describe the functional anatomy of the lower airways
The lower airways can be divided into the larynx and tracheobronchial tree, which is subdivided into the conducting and respiratory zones. Important aspects of the anatomy are:
– During inhalation, the vocal cords are in an abducted position to reduce resistance to inward gas flow.
– During exhalation, the cords adduct slightly, increasing the resistance to gas flow, which results in a positive end-expiratory pressure (PEEP) of 3–4 cmH2O. This ‘physiological’ PEEP is important for vocalisation and coughing. It also maintains positive pressure in the small airways and alveoli during expiration, thus preventing alveolar collapse and maintaining functional residual capacity (FRC).
Clinical relevance: PEEP
When a patient is intubated, the vocal cords are no longer able to adduct during exhalation, leading to a loss of physiological PEEP. This can result in atelectasis and ventilation–perfusion (V̇/Q̇) mismatch. It is common practice to apply extrinsic PEEP (PEEPe) at physiological levels (3–5 cmH2O) to maintain FRC and prevent atelectasis following intubation.
However, PEEP increases intrathoracic pressure, which increases extravascular pressure on veins, causing collapse and reducing venous return. There are a small number of situations where not applying PEEPe may be advantageous – situations where raised venous pressure may have clinical consequences. For example:
Raised intracranial pressure (ICP) – increased intrathoracic pressure may hinder venous drainage from the cerebral venous sinuses, leading to an increase in ICP.
Tonsillectomy – raised venous pressure may increase bleeding at the tonsillar bed, obstructing the surgeon’s view of the operative field.
Endotracheal and tracheostomy tubes bypass the upper airway, so the normal warming and humidification of inspired air cannot occur. Inhaling cold, dry gases results in increased mucus viscosity, which impairs the mucociliary escalator. This causes:
Accumulation of mucus in lower airways;
An increased risk of pulmonary infection;
Microatelectasis.
Artificial humidification and warming of inspired gases are commonly achieved using a heat and moisture exchanger for surgical procedures or a hot water bath humidifier in the intensive care unit.
Tracheobronchial tree:
– The tracheobronchial tree consists of a series of airways that divide, becoming progressively narrower with each division. In total, there are 23 divisions1 or generations between the trachea and the alveoli (Figure 7.1). As the generations progress, the total cross-sectional area increases exponentially (Figure 7.2).
– The tracheobronchial tree is subdivided into the conducting zone (airway generations 0–16) and the respiratory zone (generations 17–23). As the names suggest, the conducting airways are responsible for conducting air from the larynx to the respiratory zone, whilst the respiratory zone is responsible for gas exchange.
– In a 70‑kg man, the volume of the conducting airways, known as the anatomical dead space, is approximately 150 mL. The volume of the respiratory zone at rest is approximately 3000 mL.
Conducting zone:
– The first generations of airways are lined by ciliated, pseudostratified columnar epithelium with scattered goblet cells. The goblet cells secrete a mucus layer that covers the epithelial cells and traps inhaled foreign bodies or microorganisms. The cilia beat in time, propelling mucus towards the oropharynx where it is either swallowed or expectorated. This system is known as the mucociliary escalator; its function is to protect the lungs from microorganisms and particulate matter and to prevent mucus accumulation in the lower airways.
– The trachea starts at the lower border of the cricoid cartilage (C6 vertebral level) and bifurcates at the carina (T4/5 level). The anterior and lateral walls of the trachea are reinforced with ‘C’-shaped cartilaginous rings. The posterior gap of the cartilaginous rings is bridged by the trachealis muscle. At times of extreme inspiratory effort with associated high negative airway pressure, these cartilaginous rings prevent tracheal collapse.
– The trachea divides into the right and left main bronchi. The right main bronchus is shorter, wider and more vertical than the left. Inhaled foreign bodies and endotracheal tubes (ETTs) are therefore more likely to enter the right main bronchus than the left.
– The right lung has three lobes (upper, middle and lower) and the left has two lobes (upper and lower). The lingula (Latin for ‘little tongue’) is a part of the left upper lobe and is considered to be a remnant of the left middle lobe, which has been lost through evolution. There are 10 bronchopulmonary segments on the right (three upper lobe, two middle lobe, five lower lobe) and nine bronchopulmonary segments on the left (five upper lobe, four lower lobe).
Clinical relevance: double-lumen ETTs
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.
Left-sided double-lumen ETTs (DLETTs) are often favoured over right-sided tubes for one-lung ventilation, even for some right-sided thoracic surgeries. This is because incorrect positioning of a right-sided DLETT risks occlusion of the right upper lobe bronchus by the ETT cuff. Right-sided DLETTs are available and have a hole positioned for ventilation of the right upper lobe. However, there are anatomical variations in the position of the right upper lobe bronchus; the position of the DLETT and the right upper lobe bronchus should therefore be checked using fibre-optic bronchoscopy.
– In segmental and subsegmental bronchi, the epithelium is surrounded by a layer of smooth muscle. Irregularly shaped cartilaginous plates prevent airway collapse.
– The bronchioles constitute the first airway generation that does not contain cartilage. They have a layer of smooth muscle that contracts (bronchoconstriction) and relaxes (bronchodilatation) to modulate gas flow:
▪ Bronchodilatation results from sympathetic nervous system activity, such as during exercise: this reduces resistance to gas flow, allowing greater ventilation during periods of O2 demand. Drugs that induce bronchodilatation include β2-agonists and anticholinergics.
▪ Bronchoconstriction is precipitated by the parasympathetic nervous system, histamine, cold air, noxious chemicals and other factors. At rest, the reduction in gas flow velocity causes particulate material to settle in the mucus, which is then transported away from the respiratory zone by the cilia.
– The terminal bronchioles are the last (16th) airway generation of the conducting zone.
Respiratory zone:
– Respiratory bronchioles are predominantly conducting, with interspersed alveoli that participate in gas exchange. These further divide into alveolar ducts, alveolar sacs and alveoli.
– The alveoli form the final airway generation of the tracheobronchial tree. The human lungs contain approximately 300 million alveoli, resulting in an enormous surface area for gas exchange of 70 m2. Each alveolus is surrounded by a capillary network derived from the pulmonary circulation.
