Phillip is 78 and was admitted to your ward following a stroke. While he could be roused, Phillip was experiencing a reduced level of consciousness. In line with current guidance from the National Institute for Health and Care Excellence (NICE, 2019b), the ward manager undertook a rudimentary swallowing assessment by giving Phillip a teaspoon of water to drink. Unfortunately, Phillip couldn’t swallow the water, and it trickled out of the side of his mouth. Therefore, he was referred to a speech and language therapist (SALT) for a specialist assessment the next morning. Phillip was kept nil by mouth until he was assessed by the SALT.
Difficulty in swallowing (dysphagia) is common following a stroke if the glossopharyngeal nerve (cranial nerve 9) which coordinates swallowing is damaged. Failure to recognise and adequately manage dysphagia can result in aspiration pneumonia, which can potentially lead to death.
Depending on the extent of Phillip’s dysphagia, a range of measures will be considered including thickened oral fluids and nasogastric or gastrostomy feeding. Hopefully, as Phillip begins to recover from his stroke, his swallowing will improve. His recovery can be supported by the SALT.
A common mistake is to confuse the pharynx with the larynx; this confusion largely arises because these two words sound similar and are spelt in a similar manner. Remember, the term pharynx is generally used interchangeably with throat, while the larynx is the voice box.
Now that we have explored the key components of the upper respiratory tract, we need to examine the nature of the lower respiratory tract and its role in conducting air to the alveolar air sacs and in gaseous exchange.
The lower respiratory tract
The tracheobronchial tree
The airway below the vocal cords marks the beginning of the lower respiratory tract; the trachea progressively subdivides into smaller and smaller airways (Figure 4.4). The tracheobronchial tree usually consists of 23 subdivisions (generations) of the airway before the conditioned air reaches the alveolar air sacs.
The trachea
The trachea is approximately 10 to 12 cm in length and 2.5 cm in diameter. It extends from the lower border of the larynx to the carina, which marks the point where it bifurcates (splits) into the right and left primary bronchi which extend towards their respective lungs. The trachea is held open by a series of stacked C-shaped rings of cartilage which serve to reinforce and prevent the airway collapsing during inspiration and expiration. The oesophagus runs parallel to the trachea, slotting longitudinally into the long posterior groove created by the stacked C-shaped cartilaginous rings.
As in the nasal cavity and nasopharynx, the trachea is lined by a ciliated pseudostratified epithelium which sweeps contaminated mucus and particulates away from the lungs towards the pharynx to be swallowed. This mechanism is known as the mucociliary escalator because it functions in a similar manner to a mechanical stairway, continually clearing and cleaning the airway, reducing the risk of irritation and infection.
Figure 4.4 Branching structure of the respiratory tract
The carina
Located within the carina are densely arranged sensory receptors which continually monitor the airway for debris. These receptors respond to mechanical stimulation and are ideally positioned since inhaled particles will have to pass through the carina before travelling into the primary bronchi and the lungs. Once activated, these receptors initiate a vigorous coughing reflex where explosive expiration of air will usually clear irritating particulates. Unfortunately, as we grow older, the receptors of the airway become less sensitive to irritation, and this together with age-associated losses in muscle mass and strength means that the coughing reflex is less effective. This contributes to an increased risk of respiratory tract infections in older people.
The primary bronchi
The right primary bronchus is angled at approximately 20 to 30 degrees to the trachea, while the left primary bronchus is angled at between 45 and 55 degrees. This means that foreign objects entering the trachea are more likely to end up in the right bronchus and right lung than the left. As in the trachea, the primary bronchi are reinforced and held open by cartilaginous rings, but rather than being C-shaped, these are complete rings. As the bronchi subdivide, becoming smaller bronchi and bronchioles, the cartilage rings are exchanged for irregular cartilaginous plates. The right lung consists of three lobes and the left lung two (Figure 4.4).
The primary bronchi divide to form the lobar bronchi with each supplying a lobe of the lungs. The subdividing branching structure of the bronchial tree continues into each lung lobe, with each successive division (generation) of airway smaller than the one before.
Bronchioles
Bronchioles arise from the fourth generation of branching; these are small airways with a diameter of less than 1 mm and characteristically lack cartilaginous reinforcement. Structural support for the bronchioles is provided by the lung parenchyma. This is composed predominantly of the alveolar air sacs and elastic connective tissue, which attach to the external surface of these tiny airways, tethering the bronchioles (like guy ropes supporting a tent). This support is necessary to prevent airway collapse, particularly during forced inhalation and expiration.
The bronchioles themselves subdivide into smaller and smaller airways with the terminal bronchioles having a diameter of 0.5 mm or less. Smooth muscle fibres are present in the bronchiolar walls; contraction of this smooth muscle layer leads to bronchoconstriction and a narrowing of the airway. This smooth muscle is physiologically very useful since coordinated bronchoconstriction and bronchodilation can regulate airflow within the lungs. However, uncoordinated bronchoconstriction or bronchospasm can lead to air becoming trapped in the lung, which is one of the key clinical features of asthma.
The conduction zone
The airway extending from the nose all the way through the bronchial tree to the terminal bronchioles is known as the conduction zone because its role is to conduct air to the regions of the lung where gaseous exchange takes place. Jake’s asthma case study at the beginning of this chapter perfectly illustrates the importance of a clear, unobstructed conduction zone and highlights why asthma must be carefully managed to ensure an adequate supply of conditioned air to the alveolar air sacs.
The respiratory zone
Each terminal bronchiole branches into several respiratory bronchioles and alveolar ducts which finally terminate in the alveolar air sacs. Collectively, these areas have a berry-like appearance (Figure 4.5) and are referred to as acini (Latin for berries). Acini are the functional units of the lungs where gas exchange takes place. In a healthy young adult, more than 30,000 of these acini are present in each lung, with each acinus having approximately 10,000 alveoli. Around 3 litres (3000 ml) of air is held within the lung acini of a typical adult male. This contrasts with the relatively small volume of air in the conducting airways, which totals only around 150 ml; this volume is known as the anatomical dead space since the air here does not reach the alveoli to participate in gas exchange.
Figure 4.5 A small portion of a respiratory acinus, showing alveoli and associated blood capillaries
Alveoli
The alveoli (alveolus = singular) are thin-walled, sac-like structures composed of two major cell types. Type I cells are thin, flat, squamous epithelial cells which make up around 95 per cent of the alveolar wall. These cells are elastic in nature, allowing each alveolus to inflate (like a balloon) during inspiration. The alveolar wall is incredibly delicate with a width as thin as 0.2 μm, which is roughly the same thickness as the wall of a soap bubble. The remaining