more flexible section is composed of hyaline cartilage. Air enters the nose through the nostrils (external nares) and passes into the nasal cavity which is divided into left and right portions by the nasal septum.
Within the nostrils, coarse hairs called vibrissae provide the first line of defence against larger airborne particles, e.g. soot, sawdust and small insects. The primary function of the nose and nasal cavity is to condition the inspired air by adding moisture (humidification) and progressively warming it so that it is closer to the inner (core) body temperature. The turbinates or conchae are bony structures which form shelf-like projections into the nasal cavities on both sides. As air enters the nose, it diverges into the left and right nasal cavities. On reaching the turbinates, airflow becomes turbulent (swirling movement), slowing its progress and allowing air to remain in contact with the warm respiratory epithelium for longer. The nasal cavities are lined by a pseudostratified epithelium, with microscopic hair-like structures called cilia projecting from the surface.
Mucus, produced by goblet cells, is secreted onto the epithelial surface to form a protective barrier, trapping pathogens and other particulate materials. The cilia move in a coordinated sweeping motion, shifting the mucus, and any trapped particulates, to the pharynx (throat) to be swallowed or expectorated. As well as acting as a physical barrier, mucus contains immune cells, secretory antibodies and an antimicrobial enzyme called lysozyme, which rapidly breaks down bacterial cell walls. A huge network of capillaries sits just below the nasal epithelium, facilitating rapid warming of air which reaches a temperature of approximately 34°C by the time it exits the nasal cavity at the nasopharynx.
The paranasal sinuses
The paranasal sinuses are air-filled cavities in the sphenoid, ethmoid, frontal and maxilla bones of the skull. These function primarily to reduce the weight of the skull and contribute to the conditioning of the inspired air. The sinuses also act as resonance chambers amplifying vocal sounds and giving each voice its own distinctive timbre; this role becomes very apparent when we suffer a head cold with sinus congestion which frequently imparts a tinny quality to the voice.
Infection and allergic responses cause the release of inflammatory mediators such as histamine, which initiates vasodilation of the capillaries underlying the epithelium of the nasal cavity. This can result in swelling and obstruction of the narrow openings to the paranasal sinuses, causing localised pain. In some cases, the sinuses may become chronically infected, leading to a painful condition called sinusitis. Histamine can also increase mucus production in the nasal cavity, leading to a streaming nose which is a common feature of many upper respiratory tract infections and allergies.
The pharynx
The pharynx is a muscular tube running from the nasal cavity to the larynx (voice box) and is often simply referred to as the throat. It consists of three sequential sections: nasopharynx, oropharynx and laryngopharynx.
The nasopharynx is the superior portion of the pharynx, which begins at the nasal cavity. It continues the process of conditioning the inspired air and, like the nasal cavity, contains a ciliated epithelium which continues the trapping and removal of particulates. The tonsils form a ring of lymphoid tissue around the pharynx; these contain populations of immune cells called macrophages which monitor the airway and trap potential pathogens such as bacteria.
There are four groups of tonsils: the pharyngeal tonsils, or adenoids, are situated superiorly and posteriorly in the nasopharynx; the tubal tonsils sit close to the openings of the Eustachian tubes which connect to the middle ear (Chapter 6); the lingual tonsils are embedded in the root of the tongue; and the palatine tonsils are located on either side of the oropharynx. It is the almond-shaped palatine tonsils that frequently become infected and often purulent during tonsillitis.
Figure 4.2 The components of the upper respiratory tract
The middle portion of the pharynx, which lies posterior to the oral cavity, is called the oropharynx. This section is part of both the respiratory and digestive systems and is lined by a thicker, non-ciliated, stratified squamous epithelium to resist the abrasive effects of swallowing food and liquids.
The final section of pharynx is called the laryngopharynx because it is continuous with the larynx. As with the oropharynx, it is part of both the respiratory and digestive tracts and is lined by the same type of robust stratified squamous epithelium.
Since these final two sections of the pharynx form a common passageway for air, food and water, there is a danger of the airway becoming occluded should food become stuck when swallowing (deglutition). Difficulty with swallowing is termed dysphagia and this becomes more common with age, increasing the risk of choking and the aspiration of food into the airway.
Obesity is an increasing problem in the UK and is associated with upper airway dysfunction. To further your understanding of this issue, read through the following case study.
Case study: Jack – obstructive sleep apnoea
Jack is a 58-year-old man with type II diabetes who is clinically obese, weighing 115 kg and having a BMI of 33.2. He has a sedentary occupation and takes little exercise. Jack was previously a heavy smoker but gave up cigarettes following a bout of pneumonia. For some months, Jack has complained of tiredness even though he sleeps 7–8 hours a night, and he attributed this to increasing age. Jack’s wife has always teased Jack about his snoring, but over the past year it has become worse, and she worries that Jack seems to gasp and stop breathing numerous times throughout the night.
Eventually, Jack was persuaded to visit his GP, who referred him to the local hospital for sleep tests where obstructive sleep apnoea was diagnosed. Jack was provided with a Non-Invasive Positive Pressure Ventilation device (NIPPV) which pushes air under positive pressure into the airways via a close-fitting face mask, which prevents occlusion of the airways and allows Jack to sleep without airway obstruction and snoring.
In addition to getting relief from the use of positive pressure devices, patients such as Jack should be encouraged to lose weight which may eventually allow the use of such devices to be discontinued.
The larynx (voice box)
The larynx or voice box (Figure 4.3) is formed from nine pieces of fused cartilage and is connected superiorly to the laryngopharynx and inferiorly to the trachea or wind pipe. The largest of the laryngeal cartilages is the shield-shaped thyroid cartilage which is so-named because of its close proximity to the thyroid gland. The thyroid cartilage is also known as Adam’s apple since in males it usually undergoes rapid expansion during puberty and becomes more prominent.
This expansion corresponds to a general enlargement of the larynx, which usually results in the ‘breaking’ and deepening of the voice characteristic of adolescent males. Probably the most well known of the laryngeal cartilages is the epiglottis; a piece of flexible elastic cartilage which functions like a ‘trap door’ to close over the airway during swallowing to prevent aspiration of food and fluids into the lungs (Chapter 10).
Figure 4.3 Structure of the larynx
The cricoid cartilage lies inferior to and is separated from the thyroid cartilage by the cricothyroid membrane. If the airway is occluded and the patient cannot breathe, an incision can be made in the cricothyroid membrane and a tube inserted to allow artificial ventilation to be established; this procedure is an example of intubation. Inside the larynx are the vocal cords; these are tiny ligaments that vibrate when air is passed over them to generate sound. The tension of the vocal cords can be varied by muscles to alter the pitch of sound produced; this effect is similar to what happens when tightening and loosening a guitar string.
Case study: Phillip – dysphagia