Specialised epithelial cells involved in detecting smells, olfactory epithelial cells, lie in the uppermost parts of the nasal cavity. Thus, sniffing odours into the upper part of the nasal cavity is the most efficient way of detecting smells.

Lying adjacent to the nasal cavity are paranasal sinuses. They are four paired structures (the maxillary, frontal, sphenoid and ethmoid sinuses) which open into the nasal cavity. These sinuses are air-filled spaces in skull bones which are lined by respiratory epithelium. The function of the sinuses is unclear, though it may be related to insulating the brain from the effects of inspiring cold air. Other suggested functions include giving resonance to the voice, lightening the weight of the skull and adding to the ability of the upper respiratory tract to ‘air condition’ (warm and moisten) inhaled air.

In the nasal cavities and the sinuses the mucosa and associated blood vessels are firmly attached to underlying bone or cartilage, and the rigidity conferred by such attachments means that inspiration does not collapse the air passages in these regions.

The nasopharynx and the oropharynx are contiguous parts of the pharynx (a large passage shared by the respiratory and the digestive systems). The nasopharynx, which is traversed by air, is lined by respiratory epithelium, but the oropharynx, which carries food (and drink and air) from the mouth, is lined by a stratified squamous epithelium which is able to resist the ‘wear and tear’ caused by the passage of food.

Aggregations of lymphocytes are a prominent feature deep to the epithelium lining the nasopharynx, particularly on its posterior wall where they form the nasopharyngeal tonsils (adenoids). These aggregations form part of a ring of lymphoid cells (Waldeyer’s ring) around the pharynx. Inhaled and ingested antigenic material (e.g. bacteria and viruses) may be trapped in this region and immune responses mounted, thus protecting the respiratory and gastrointestinal tracts from infection.

Inspired air passes from the nasopharynx into the larynx and then into the trachea. The larynx has walls containing hyaline cartilage which maintain the patency of the airway, and most of the lumen of the larynx is lined by respiratory epithelium. However, there are flaps (the vocal folds) extending from the walls of the larynx into the lumen which are covered by a stratified squamous epithelium. Vocal cords vibrate and produce sounds and the stratified squamous epithelium resists the wear and tear resulting from the vibrations.

During swallowing, the entry to the larynx from the pharynx is closed temporarily by a large flap-like structure, the epiglottis. This arrangement prevents ingested food and liquid entering the trachea and producing coughing and choking. The stratified squamous epithelium covering the anterior surface of the epiglottis is continuous with the dorsal surface of the tongue and, as it is in contact with food during swallowing is subject to abrasion. In contrast, the posterior surface of the epiglottis is exposed only to air and is covered by respiratory epithelium. The flexibility and recoil movements of the epiglottis during swallowing are aided by its core of elastic cartilage (see Fig. 9.1).

The trachea is a tube attached to the larynx in the neck and it extends about 10cm into the thorax. It is 2–3cm in diameter and is kept patent by 15 to 20 incomplete ‘C’-shaped rings of hyaline cartilage in its wall (Fig. 11.3). Each ring of cartilage is completed on the posterior wall of the trachea by smooth muscle and connective tissue containing elastin fibres. The trachea is lined by respiratory epithelium (Fig. 11.3) and there are serous and mucous glands (Fig. 11.4) in submucosal connective tissue. Secretions from the submucosal glands and goblet cells in the epithelium are propelled by ciliary activity toward the pharynx and are usually swallowed.

The trachea bifurcates into two main (primary) bronchi (Fig. 11.1). These enter the lungs and divide into lobar (secondary) bronchi. Each of these subdivides about 10 times into segmental (tertiary) bronchi which supply specific areas of lung known as bronchopulmonary segments. Each of these segments of lung has its own branch of a pulmonary artery as well as a single tertiary bronchus, and each has clinical significance. For example, knowledge of the architecture of bronchopulmonary segments is vital in bronchoscopy. In addition, a disease may be confined to one segment of lung and surgical resection may be used to remove the diseased segment. Further branching of bronchi in segments reduces their diameter to 5mm and, at this dimension, branching continues but the passages are then known as bronchioles.

All bronchi contain hyaline cartilage in their walls which maintains their patency. Primary bronchi have, like the trachea, incomplete rings of cartilage in their walls. Intrapulmonary branches of bronchi have irregularly placed plates of cartilage in their walls (Fig. 11.5). Large bronchi are lined by respiratory epithelium (Fig. 11.5), but as they become smaller in diameter gradual changes occur in the epithelium (see below). The submucosal layer of bronchi contains serous and mucous glands which secrete onto the epithelial surface (Fig. 11.6). Secretions on the epithelial surface are moved up the ‘mucus escalator’ to the pharynx by ciliary activity of the columnar epithelial cells. Smooth muscle (Figs 11.2 and 11.6) is also present in the walls of bronchi and the muscle cells are arranged spirally along the length of the walls. Parasympathetic stimulation of this smooth muscle causes contraction which reduces the diameter of the lumen. Conversely, sympathetic nerve stimulation relaxes the muscle and increases the diameter of the airway. The strength of muscle contraction is not great enough to collapse the bronchi because of the relatively rigid cartilage lying outside the muscle layer. Aggregations of lymphocytes are a common feature in the walls of bronchi (Figs 11.2 and 11.5) and are described as ‘bronchus-associated lymphoid tissue’ (BALT). In these regions, interactions between macrophages and lymphocytes take place which are important in immune defence of the lungs.

Bronchioles, passages of less than 5mm in diameter, do not have cartilage in their walls but have smooth muscle (Fig. 11.7). The patency of bronchioles during inspiration is assisted not by cartilage in their walls but by elastin fibres in surrounding connective tissue attached to the bronchioles which are stretched as the thoracic volume is increased during inspiration and air is drawn into the respiratory tract.

As the conducting tubes of the respiratory system reduce in calibre, the type of lining epithelium gradually changes. Instead of the typical respiratory epithelium (pseudostratified, ciliated columnar, with goblet cells) as in the larger bronchi (Figs 11.2, 11.5 and 11.6), they become lined by a simple epithelium. Most bronchioles are lined by a simple epithelium consisting of ciliated columnar cells and a few goblet cells. Smaller divisions of bronchioles are lined by ciliated low columnar or cuboidal cells (Fig. 11.7); goblet cells are not present in the smaller bronchioles. In terminal bronchioles the epithelium may become flattened taking on a squamous appearance (Fig. 11.8). There are also specialised cells (Clara cells) in the epithelium lining the smallest bronchioles which may serve a protective function. (Clara cells are prominent in some non-human species.)

There are no submucous glands in bronchioles and very few goblet cells. If such glands and goblet cells were present, it could lead to too much fluid in the bronchioles which may drain into and ‘drown’ the gaseous exchange region of the lower respiratory tract.