The function of the respiratory system is to transport gases between the atmosphere and sites in the lungs where gaseous exchange between air and blood occurs. Oxygen diffuses into blood in capillaries in the lungs and carbon dioxide is released from the blood. The respiratory system consists of a series of air-filled passages connecting the nose and mouth to the two lungs in the thorax (Fig. 11.1). Two categories of passages are described, the upper and the lower respiratory tracts. The structure of the walls of the passages of the upper respiratory tract ensures that they do not collapse during breathing. The structure of the walls of the lower respiratory tract ensures that efficient gaseous exchange occurs across a barrier which is 0.1–1.5μm thick. It is also essential that the passages in the lower respiratory tract do not collapse during respiration.
Whilst the respiratory passages conduct gases through the respiratory system, there are other related structures that help propel the gases along the passages. Each lung is enclosed by a pleural sac formed by a serosal membrane (Fig. 11.1). The visceral layer of the pleura is firmly attached to the surface of each lung and the parietal layer to the inner surface of the chest wall. The pleural cavity (the space enclosed by the pleura) contains fluid secreted by the serosal cells and the lungs are thus able to move during respiration in a relatively friction-free environment. The pleural cavity has an important role in breathing as the pressure in the cavity is less than atmospheric pressure. As the thoracic boundaries (the ribs, intercostal muscles and diaphragm) move during breathing the pleural membranes and lungs move with them. Inspiration increases intrathoracic volume (and the negative pressure in the pleural cavities) and draws air into the lungs. Expiration occurs as the thoracic boundaries decrease intrathoracic volume and stretched elastin fibres in the lung (see below) recoil.
Upper respiratory tract
The upper respiratory tract comprises passages and tubes of decreasing diameter which connect the nose and mouth to the lower respiratory tract in the lungs. From the exterior inwards, the upper respiratory tract comprises the nasal cavity, nasopharynx, larynx, trachea, bronchi and some bronchioles. These passages are also known collectively as the conducting portion of the respiratory tract as they conduct air to the sites of gaseous exchange in the lungs.
During inspiration the upper respiratory passages are under increasing negative pressure as the intrathoracic volume increases. Air will be drawn into the lungs only if the walls of these passages do not collapse. The larger passages have bone or cartilage in their walls which make them relatively rigid and ensure that they remain patent during inspiration. The smallest passages within the lungs, the bronchioles, do not have bone or cartilage in their walls. Bronchioles are held open during inspiration as elastin connective tissue fibres attached to the outer surface of their walls are stretched as a result of the thoracic volume increasing during inspiration.
Most of the upper respiratory tract is lined by a mucosa which consists of a respiratory epithelium and a lamina propria which supports numerous blood vessels. In some regions submucosal connective tissue and aggregations of lymphoid cells are also present. The respiratory epithelium is described as pseudostratified with ciliated columnar epithelial cells and goblet cells (Fig. 11.2). The goblet cells secrete mucus onto the surface of the epithelium and this traps particulate matter which may be harmful. Importantly, the beating motion of cilia of columnar epithelial cells moves the mucus so that it is swallowed or discharged from the nose or the mouth. In addition, moisture on the surface of the respiratory epithelium, provided by mucus, humidifies inspired air. This prevents dehydration of the cells lining the lower respiratory tract where gases have to enter an aqueous phase to allow exchange between air and blood. The respiratory epithelium also contains stem cells which can replace damaged goblet and ciliated epithelial cells and replace themselves. Neuroendocrine cells are also present which secrete molecules (paracrine hormones) that regulate the local environment.
Nasal cavities and sinuses
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.
Nasopharynx
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.
Larynx
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.
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