The pleura and airways

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The respiratory tract is most often discussed in terms of upper and
lower parts. The upper respiratory tract relates to the nasopharynx and
larynx whereas the lower relates to the trachea, bronchi and lungs.
The pleurae
• Each pleura consists of two layers: a visceral layer which is adherent
to the lung and a parietal layer which lines the inner aspect of the chest
wall, diaphragm and sides of the pericardium and mediastinum.
• At the hilum of the lung the visceral and parietal layers become continuous.
This cuff hangs loosely over the hilum and is known as the pulmonary
ligament. It permits expansion of the pulmonary veins and
movement of hilar structures during respiration (Fig. 5.1).
• The two pleural cavities do not connect.
• The pleural cavity contains a small amount of pleural fluid which acts
as a lubricant decreasing friction between the pleurae.
• During maximal inspiration the lungs almost fill the pleural cavities.
In quiet inspiration the lungs do not expand fully into the costodiaphragmatic
and costomediastinal recesses of the pleural cavity.
• The parietal pleura is sensitive to pain and touch (carried by the intercostal
and phrenic nerves). The visceral pleura is sensitive only to
stretch (carried by autonomic afferents from the pulmonary plexus).
Air can enter the pleural cavity following a fractured rib or a torn
lung (pneumothorax). This eliminates the normal negative pleural
pressure, causing the lung to collapse.
Inflammation of the pleura (pleurisy) results from infection of the
adjacent lung (pneumonia). When this occurs the inflammatory process
renders the pleura sticky. Under these circumstances a pleural rub can
often be auscultated over the affected region during inspiration and
expiration. Pus in the pleural cavity (secondary to an infective process)
is termed an empyema.
The trachea (Fig. 5.2)
• Course: the trachea commences at the level of the cricoid cartilage in
the neck (C6). It terminates at the level of the angle of Louis (T4/5)
where it bifurcates into right and left main bronchi.
• Structure: the trachea is a rigid fibroelastic structure. Incomplete
rings of hyaline cartilage continuously maintain the patency of
the lumen. The trachea is lined internally with ciliated columnar
epithelium.
• Relations: behind the trachea lies the oesophagus. The 2nd, 3rd and
4th tracheal rings are crossed anteriorly by the thyroid isthmus (Figs 5.3
and 64.1).
• Blood supply: the trachea receives its blood supply from branches of
the inferior thyroid and bronchial arteries.
The bronchi and bronchopulmonary segments (Fig. 5.2)
• The right main bronchus is shorter, wider and takes a more vertical
course than the left. The width and vertical course of the right main
bronchus account for the tendency for inhaled foreign bodies to preferentially
impact in the right middle and lower lobe bronchi.
• The left main bronchus enters the hilum and divides into a superior
and inferior lobar bronchus. The right main bronchus gives off the
bronchus to the upper lobe prior to entering the hilum and once into the
hilum divides into middle and inferior lobar bronchi.
• Each lobar bronchus divides within the lobe into segmental bronchi.
Each segmental bronchus enters a bronchopulmonary segment.
• Each bronchopulmonary segment is pyramidal in shape with its apex
directed towards the hilum (see Fig. 6.1). It is a structural unit of a lobe
that has its own segmental bronchus, artery and lymphatics. If one
bronchopulmonary segment is diseased it may be resected with preservation
of the rest of the lobe. The veins draining each segment are
intersegmental.
Bronchial carcinoma is the commonest cancer among men in the
United Kingdom. Four main histological types occur of which small
cell carries the worst prognosis. The overall prognosis remains
appalling with only 10% of sufferers surviving 5 years. It occurs most
commonly in the mucous membranes lining the major bronchi near the
hilum. Local invasion and spread to hilar and tracheobronchial nodes
occurs early.