The thoracic cavity is divided into three - the right and left pleural cavities, each containing a lung, and the central mediastinum, containing the heart within its
pericardium and the great vessels, trachea, thymus gland, and other structures in transit to the abdomen (oesophagus, vagus and phrenic nerves, and thoracic duct).
PLEURAL CAVITIES
Left and right pleural cavities are entirely separate from each other. Each (Fig. 3.18) is formed as the developing lung bud invades a space (part of the coelomic cavity, see page 43) by pushing before it a layer of the wall (just as a finger may invaginate a partially inflated balloon). The structures involved are the lung bud (finger), inner layer of pleura (nearer wall of balloon), pleural cavity (air space), outer layer of pleura (far wall of balloon).
The pleura may thus be divided into three parts:
1. parietal layer, lining the thoracic wall, the thoracic side of the diaphragm and the medial wall of the mediastinum;
2. visceral layer, closely covering all surfaces of the lungs;
3. a junctional region, the pleural cuff surrounding the hilus or root of the lung where the two layers join. The pleural cuff has some slack, leading to the formation of a fold called the pulmonary ligament, to allow relative movement of the lung and mediastinum.
Fig. 3.18. Development of the pleural cavities.
The pleural cavity is located between the two layers of pleura and is normally a potential space only over most of the lung surface as the visceral and parietal pleurae are in virtual contact, being separated by only a thin layer of watery lubricating pleural fluid.
The parietal pleura is often arbitrarily divided into costal, diaphragmatic, and mediastinal areas; this is useful when considering their nerve supply. The costal pleura is supplied
segmentally by intercostal nerves, the mediastinal pleura by the phrenic and the diaphragmatic pleura centrally by the phrenic and peripherally by the lower five
intercostal nerves. The visceral pleura receives only an autonomic vasomotor supply, and is insensitive to pain and touch.
Clinical aspects
Disease states of the lung (e.g. pneumonia) are often painless until the parietal pleura becomes involved. Local infection of the pleural cavity may lead to a breakdown of
lubrication between pleural layers which may then adhere due to the formation of fibrous scar tissue. Local breakdown of lubrication can give a painful and (through a stethoscope) audible pleural rub. General infection of the pleura (pleurisy) is a painful condition, the pain often being referred to the cutaneous distribution of the segmental nerves supplying the pleura. (Interestingly, pleural pain may be abolished by applying local anaesthetic to the appropriate skin area.)
Adhesions may be deliberately induced, by placing a slightly irritant powder in the pleural cavity, to cure chronic leakage of air from a diseased lung.
The pleural cavity may become infiltrated by:
1. Air (pneumothorax). This usually enters via a diseased lung, much less commonly by a stab wound through the chest wall. Spontaneous pneumothorax may occur in the healthy lung after great exertion. The most dangerous type of pneumothorax is where the ruptured lung has a valvular effect, allowing air to pass from the lung to pleural cavity but not vice versa. This is a tension pneumothorax and leads to a progressive build-up of pressure in the pleural cavity and hence collapse of the lung. The effects include breathlessness and,
because the mediastinal structures are displaced towards the opposite side, pain.
2. Excess serous pleural fluid (pleural effusion). This will also affect the efficiency of the lung and displace the mediastinum. Infection of this fluid is common.
3. Pus (empyaema).
4. Blood (haemothorax).
Fluid is usually aspirated from the pleural cavity by means of a pleural tap, a wide bore needle being inserted through a lower intercostal space so as to pass into the pleural cavity beneath the lung but above the diaphragm. Pus from a lung abscess, blood, or air is usually aspirated locally after ascertaining its exact position on X-ray.
TRACHEA
The trachea (Fig. 3.19) is a tube about 12.5 cm long and 2.5 cm in diameter running from the lower border of the cricoid cartilage in the neck to its bifurcation into left and right main bronchi at the level of the sternal angle. It is stiffened by U-shaped rings of cartilage, incomplete posteriorly to allow the passage of food boli in its posterior relation, the
oesophagus. The wall between the rings is made up of fibrous tissue and the lumen lined by respiratory (ciliated columnar) epithelium. The posterior wall of the trachea contains a considerable amount of smooth muscle.
Fig. 3.19. The trachea and its relationships.
The trachea shares the upper mediastinum with the following structures-anteriorly: the thymus gland, brachiocephalic veins, arch of the aorta and origin of the brachiocephalic artery; posteriorly: the oesophagus and left recurrent laryngeal nerve; right side: the azygos vein and right vagus and phrenic nerves; left side: the left common carotid, left subclavian artery and left vagus and phrenic nerves.
The right main bronchus is shorter, wider and more vertical than the left. About 21/2 cm long, it gives off a branch to the upper lobe before entering the right lung. The left main bronchus is about 5 cm long and passes to the left and downwards below the arch of the aorta, in front of the oesophagus to enter the left lung. Right and left main bronchi are subdivided further within the substance of the lung, each lobe having its own lobar bronchus.
The structure of the walls of the bronchi is similar to that of the trachea although the cartilages become increasingly irregular below the bifurcation.
Clinical aspects
Despite the cartilaginous rings, an enlarged thyroid gland or dilated aortic arch (aneurism) may compress the trachea with resultant difficulty in breathing.
Inflammation of the trachea (tracheitis) or bronchi (bronchitis) gives a burning sensation referred to the sternum.
Foreign bodies entering the larynx and trachea will stimulate a powerful cough reflex.
None the less, small toys, peanuts, and a wide range of other objects are not uncommonly inhaled by small children. The cough reflex is in abeyance in the unconscious subject so that blood and vomit may enter the lungs in the victims of road accidents and other
violence. When a patient is given a general anaesthetic for dental extraction, procedures (such as packing the throat or passing an endotracheal tube) must be adopted to ensure that tooth fragments and other foreign bodies are not inhaled. The ridge (carina) formed by the bifurcation of the trachea is the last area to contain sensory nerve endings capable of eliciting the cough reflex and may thus be considered a last line of defence. When
foreign bodies do pass the carina, they usually enter the right main bronchus and thence
the middle and lower bronchi of the right lung. An inhaled foreign body may block one of the bronchi leading to collapse of the lung or part of it or, if infected, may cause a lung abscess.
Carcinoma of the bronchus is the most common form of cancer, responsible for 30 per cent of malignancies. Because of the arrangement of the lymphatic drainage, it may spread to the pleura and to lymph nodes at the hilus of the lung and in the mediastinum. From these lymph nodes the tumour soon metastasizes to more distant organs (especially brain).
Pressure or invasion from an enlarged hilar lymph node may involve the phrenic nerve with paralysis of the corresponding half of the diaphragm, the recurrent laryngeal nerve causing hoarseness or the adjacent sympathetic trunk producing Horner's syndrome (see p. 253).
The interior of the bronchi as far as the main branches of the lobar bronchi can be viewed by means of a bronchoscope passed through the mouth and larynx. A bronchoscope can also be used to take samples of mucosa for examination, to remove foreign bodies and to
aspirate accumulations of fluid.
Tracheostomy (the provision of an opening in the anterior wall of the trachea) may be necessary to bypass laryngeal obstruction (foreign bodies, tumours) or to drain copious secretions (in a post-operative patient too weak to cough) or for long-term artificial respiration (poliomyelitis).
LUNGS
Both lungs are conical, covered by visceral pleura (which adheres tightly to their surfaces) and attached to the mediastinum by a lung root. Each has a blunt apex projecting about 21/2cm above the medial third of the clavicle, a concave base over the diaphragm, an extensive costal surface and a concave mediastinal surface. In fixed post mortem
specimens evidence of lung relations in the form of grooves moulded around the aorta, ribs, etc. can be seen. These are, of course, not
present in the living, where the lung has a very light frothy texture, rather like foam rubber.
Fig. 3.20. Lobes and bronchopulmonary segments of the lungs.
The right lung is slightly larger than the left (because of the offset of the heart which decreases the size of the left pleural cavity). Both are divided by an oblique fissure (Fig.
3.20). The upper part of the right lung is subdivided by a horizontal fissure into upper and middle lobes. The left lung has a recess, the cardiac notch, in its medial side which
accommodates the heart. Below the notch the upper lobe may be extended into a lingula.
Each lung is further functionally divided into a number of pyramidal bronchopulmonary segments whose bases are illustrated in Fig. 3.20. These are of interest to the physician and thoracic surgeon as they form functional units supplied by segmental bronchi and blood vessels. A single segment may, therefore, be removed with minimal loss of blood or
leakage of air.