The Thorax-Lower Respiratory Tract

The trachea (Figs 14, 15)

The trachea is about 11.5cm length and nearly 2.5cm diameter. It commences at the lower border of the cricoid cartilage (C6) and terminates by bifurcating at the level of the sternal angle of Louis (T4/5) to form the right and left main bronchi. (In the living subject, the level of bifurcation varies slightly with the phase of respiration; in deep inspiration it descends to T6 and in expiration it rises to T4.)

Relations

Lying partly in the neck and partly in the thorax, its relations are:

Cervical

•◊◊anteriorly— the isthmus of thyroid gland, inferior thyroid veins, sternohyoid and sternothyroid muscles;

•◊◊laterally—the lobes of thyroid gland and the common carotid artery;

•◊◊posteriorly—the oesophagus with the recurrent laryngeal nerve lying in the groove between oesophagus and trachea (Fig. 16).

Thoracic

In the superior mediastinum its relations are:

•◊◊anteriorly—commencement of the brachiocephalic (innominate) artery

and left carotid artery, both arising from the arch of the aorta, the left brachiocephalic

(innominate) vein, and the thymus;

•◊◊posteriorly—oesophagus and left recurrent laryngeal nerve;

•◊◊to the left— arch of the aorta, left common carotid and left subclavian

arteries, left recurrent laryngeal nerve and pleura;

•◊◊to the right—vagus, azygos vein and pleura (Fig. 17).

 Structure

The patency of the trachea is maintained by a series of 15–20 U-shaped cartilages. Posteriorly, where the cartilage is deficient, the trachea is flattened and its wall completed by fibrous tissue and a sheet of smooth muscle (the

trachealis). Within, it is lined by a ciliated columnar epithelium with many goblet cells.

Clinical features

Radiology

Since it contains air, the trachea is more radio-translucent than the neighbouring structures and is seen in posteroanterior and lateral radiographs as a dark area passing downwards, backwards and slightly to the right. In the elderly, calcification of the tracheal rings may be a source of radiological confusion.

Displacement

The trachea may be compressed or displaced by pathological enlargement of the neighbouring structures, particularly the thyroid gland and the arch of the aorta.

‘Tracheal-tug’

The intimate relationship between the arch of the aorta and the trachea and left bronchus is responsible for the physical sign known as ‘tracheal-tug’, characteristic of aneurysms of the aortic arch.

Tracheostomy

Tracheostomy may be required for laryngeal obstruction (diphtheria, tumours, inhaled foreign bodies), for the evacuation of excessive secretions (severe postoperative chest infection in a patient who is too weak to cough adequately), and for long-continued artificial respiration (poliomyelitis, severe chest injuries). It is important to note that respiration is further assisted by considerable reduction of the dead-space air. The neck is extended and the head held exactly in the midline by an assistant. A vertical incision is made downwards from the cricoid cartilage, passing between the anterior jugular veins.

Alternatively, a more cosmetic transverse skin crease incision, placed halfway between the cricoid and suprasternal notch, is employed. A hook is thrust under the lower border of the cricoid to steady the trachea and pull it forward. The pretracheal fascia is split longitudinally, the isthmus of the thyroid either pushed upwards or divided between clamps and the cartilage of the trachea clearly exposed. A circular opening is then made into the trachea to admit the tracheostomy tube. In children the neck is relatively short and the left brachiocephalic vein may come up above the suprasternal notch so that dissection is rather more difficult and dangerous. This difficulty is made greater because the child’s trachea is softer and more mobile than the adult’s and therefore not so readily identified and isolated. Its softness means that care must be taken, in incising the child’s trachea, not to let the scalpel plunge through and damage the underlying oesophagus.

In contrast, the trachea may be ossified in the elderly and small bone shears required to open into it. The golden rule of tracheostomy—based entirely on anatomical considerations— is ‘stick exactly to the midline’. If this is not done, major vessels are in jeopardy and it is possible, although the student may not credit it, to miss the trachea entirely.

The bronchi (Fig. 15)

The right main bronchus is wider, shorter and more vertical than the left. It is about 2.5cm long and passes directly to the root of the lung at T5. Before joining the lung it gives off its upper lobe branch, and then passes below the pulmonary artery to enter the hilum of the lung. It has two important relations: the azygos vein, which arches over it from behind to reach the superior vena cava, and the pulmonary artery which lies first below and then anterior to it. The left main bronchus is nearly 2.5cm long and passes downwards and outwards below the arch of the aorta, in front of the oesophagus and descending aorta. Unlike the right, it gives off no branches until it enters the hilum of the lung, which it reaches opposite T6. The pulmonary artery spirals over the bronchus, lying first anteriorly and then above it.

Clinical features

1◊◊The greater width and more vertical course of the right bronchus accounts for the greater tendency for foreign bodies and aspirated material to pass into the right bronchus (and thence especially into the middle and lower lobes of the right lung) rather than into the left.

2◊◊The inner aspect of the whole of the trachea, the main and lobar bronchi and the commencement of the first segmental divisions can be seen at bronchoscopy.

3◊◊Widening and distortion of the angle between the bronchi (the carina) as seen at bronchoscopy is a serious prognostic sign, since it usually indicates carcinomatous involvement of the tracheobronchial lymph nodes around the bifurcation of the trachea.

The lungs (Figs 18, 19)

Each lung is conical in shape, having a blunt apex which reaches above the sternal end of the 1st rib, a concave base overlying the diaphragm, an extensive costovertebral surface moulded to the form of the chest wall and a mediastinal surface which is concave to accommodate the pericardium.

The right lung is slightly larger than the left and is divided into three lobes—upper, middle and lower, by the oblique and horizontal fissures. The left lung has only an oblique fissure and hence only two lobes.

Blood supply

Mixed venous blood is returned to the lungs by the pulmonary arteries; the air passages are themselves supplied by the bronchial arteries, which are small branches of the descending aorta. The bronchial arteries, although small, are of great clinical importance. They maintain the blood supply to the lung parenchyma after pulmonary embolism, so that, if the patient recovers, lung function returns to normal. The superior and inferior pulmonary veins return oxygenated blood to the left atrium, while the bronchial veins drain into the azygos system.

Lymphatic drainage

The lymphatics of the lung drain centripetally (MOVING TOWARD THE CENTER) from the pleura towards the hilum. From the bronchopulmonary lymph nodes in the hilum, efferent lymph channels pass to the tracheobronchial nodes at the bifurcation of the trachea, thence to the paratracheal nodes and the mediastinal lymph trunks to drain usually directly into the brachiocephalic veins or, rarely, indirectly via the thoracic or right lymphatic duct.

Nerve supply

The pulmonary plexuses derive fibres from both the vagi and the sympathetic trunk. They supply efferents (refers to nerves that travel from the brain and spinal cord to the rest of the body) to the bronchial musculature (sympathetic bronchodilator fibres) and receive afferents (NERVE FIBRES) from the mucous membrane of the bronchioles and from the alveoli.

The bronchopulmonary segments of the lungs (Figs 20, 21) 

A knowledge of the finer arrangement of the bronchial tree is an essential prerequisite to intelligent appreciation of lung radiology, to interpretation of bronchoscopy and to the surgical resection of lung segments. Each lobe of the lung is subdivided into a number of bronchopulmonary segments, each of which is supplied by a segmental bronchus, artery and vein.

These segments are wedge-shaped with their apices at the hilum and bases at the lung surface; if excised accurately along their boundaries (which aremarked by intersegmental veins), there is little bleeding or alveolar air leakage from the raw lung surface.

The names and arrangements of the bronchi are given in Table 1; each bronchopulmonary segment takes its title from that of its supplying segmental bronchus (listed in the right-hand column of the table). The left upper lobe has a lingular segment, supplied by the lingular bronchus from the main upper lobe bronchus. This lobe is equivalent to the right middle lobe whose bronchus arises as a branch from the main bronchus.

Apart from this, differences between the two sides are very slight; on the left, the upper lobe bronchus gives off a combined apicoposterior segmental bronchus and an anterior branch, whereas all three branches are separate on the right side.

On the right also there is a small medial (or cardiac) lower lobe bronchus which is absent on the left, the lower lobes being otherwise mirror images of each other.