Structure of the respiratory system
The anterior one-third of the nasal cavity is divided into right and left halves by the nasal septum. The nasal vestibule leads to the internal ostium (a) which is the narrowest part of the nasal cavity. This causes a 50% increased resistance to airflow when breathing through the nose rather than through the mouth. The respiratory region (b) is divided by three folds arising from the lateral wall, termed the superior, middle and inferior turbinates. Behind these turbinates are situated the openings of the nasolacrimal duct and the frontal, ethmoidal and maxillary sinuses. The olfactory region for smell is found above the superior turbinate. The nasal cavities communicate with the nasopharynx via the posterior nasal apertures (the choanae (c)), and the eustachian tube opens into this area just above the soft palate.
The pharynx and larynx
The pharynx is divided by the soft palate into an upper nasopharyngeal and lower oropharyngeal region. There are numerous collections of lymphoid tissue arranged in a circular fashion around the nasopharynx; these include the adenoids. The tonsils lie between the anterior and posterior fauces, separating the mouth from the oropharynx.
The larynx consists of a number of articulated cartilages, vocal cords, muscles and ligaments, all of which serve to keep the airway open during breathing and occlude it during swallowing. The main motor nerve to the larynx is the recurrent laryngeal nerve. The left recurrent laryngeal nerve leaves the vagus at the level of the aortic arch, hooking round it to run upwards through the mediastinum between the trachea and the oesophagus; it can be affected by disease in these areas. The principal tensor of the vocal cords is the external branch of the superior laryngeal nerve, which can be injured during thyroidectomy. The trachea, bronchi and bronchioles
The trachea is 10-12 em in length. It lies slightly to the right of the midline and divides at the carina into right and left main bronchi. The carina lies under the junction of the manubrium sternum and the second right costal cartilage. The right main bronchus is more vertical than the left and, hence, inhaled material is more likely to pass into it.
The right main bronchus divides into the upper lobe bronchus and the intermediate bronchus, which further subdivides into the middle and lower lobe bronchi. On the left the main bronchus divides into upper and lower lobe bronchi only. Each lobar bronchus further divides into segmental and sub segmental bronchi. There are about 25 divisions in all between the trachea and the alveoli. Of the first seven divisions the bronchi have:
• Walls consisting of cartilage and smooth muscle
• Epithelial lining with cilia and goblet cells
• Submucosal mucus-secreting glands
• Endocrine cells-Kulchitsky or APUD (amine precursor and uptake decarboxylation) containing 5- hydroxytryptamine
In the next 16-18 divisions the bronchioles have:
• No cartilage and a muscular layer that progressively becomes thinner
• A single layer of ciliated cells but very few goblet cells
• Granulated Clara cells that produce a surfactant-like substance
The ciliated epithelium is an important defence mechanism. Each cell contains approximately 200 cilia beating at 1000 min-I in organized waves of contraction. Each cilium consists of nine peripheral parts and two inner longitudinal fibrils in a cytoplasmic matrix. Nexin links join the peripheral pairs. Dynein arms consisting of ATPase protein project towards the adjacent pairs. Bending of the cilia results from a sliding movement between adjacent fibrils powered by an ATP-dependent shearing force developed by the dynein arms. Absence of dyne in arms leads to immotile cilia. Mucus, which contains macrophages, cell debris, inhaled particles and bacteria, is moved by the cilia towards the larynx at about 1.5 em min-I-the ‘mucociliary escalator’ (see below). The bronchioles finally divide within the acinus into smaller respiratory bronchioles that have alveoli arising from the surface. Each respiratory bronchiole supplies approximately 200 alveoli via alveolar ducts. The term ‘small airways’ refers to bronchioles of less than 2 mm; there are 30000 of these in the average lung.
There are approximately 300 million alveoli in each lung. Their total surface area is 40-80 m”, The epithelial lining consists largely of type I pneumocytes. These cells have an extremely attenuated cytoplasm, and thus provide only a thin barrier to gas exchange. They are derived from type IIpneumocytes. Type I cells are connected to each other by tight junctions that limit the fluid movements in and out of the alveoli.
Type IIpneumocytes are slightly more numerous than type Icells but cover less of the epithelial lining. They are found generally in the borders of the alveolus and contain distinctive lamellar vacuoles, thought to be a source of surfactant. Macrophag~s are also present in the alveoli and are involved in the defence mechanisms of the lung. The pores of Kohn are holes in the alveolar wall allowing communication between alveoli of adjoining lobules.
The lungs are separated into lobes by invaginations of the pleura, which are often incomplete. The right lung has three lobes, whereas the left lung has two. The position of the oblique fissures and the right horizontal fissure are shown. The upper lobe lies mainly in front of the lower lobe and therefore signs on the right side in the front of the chest found on physical examination are due to lesions mainly of the upper lobe or part of the middle lobe.
Each lobe is further subdivided into bronchopulmonary segments by fibrous septa that extend inwards from the pleural surface. Each segment receives its own segmental bronchus.
The bronchopulmonary segment is further divided into individual lobules approximately 1 cm in diameter and generally pyramidal in shape, the apex lying towards the bronchioles supplying them. Within each lobule a terminal bronchus supplies an acinus and within this structure further divisions of the bronchioles eventually give rise to the alveoli.
A chest X-ray illustrates the above features.