Chapter 8 Respiratory System
Upper Respiratory Tract
Acute Inflammation
Infections of the nose, nasal sinuses, pharynx and larynx are common. They are usually mild and self-limiting. Most cases are due to viral infection, but this is often followed by bacterial superinfection.
This phase is characterised by features of acute inflammation but without the exudation of neutrophils.
A wide variety, of which the major types are:
The wide variety of different viruses involved prevents protective immunity.
Rhinitis
Common Cold (Acute Coryza)
This common respiratory inflammation usually involves the nose and adjacent structures.
The 2 phases, viral and bacterial, are typically seen in this disease.
The drainage from the sinuses, especially the maxillary, is often blocked by swelling of the mucosa — giving rise to sinusitis.
The infection is acquired by droplet spread of viruses by sneezing.
Allergic Rhinitis ‘Hay Fever’
An allergic (immediate type hyper-sensitivity) type of inflammation (2. p.101) is often seen. Patients develop immediate symptoms of sneezing, itching and watery rhinorrhoea.
The repeated attacks frequently lead to chronic changes in the mucosa with polyp formation.
Nasal Polyps
These form particularly on the middle turbinate bones and within the maxillary sinuses.
In the nasopharynx, the lymphoid tissue may be greatly enlarged (adenoids) and contribute to the nasal obstruction.
Vasomotor Rhinitis
This is a clinical diagnosis. The pathological changes are, to some extent, similar to those of allergic rhinitis, but the condition is more continuous, less spasmodic. Although the cause is unknown, viral infections in a polluted atmosphere (usually an urban setting) are thought to initiate the ‘sensitivity’ and non-specific stimuli such as bright light and smells precipitate an attack.
Acute Pharyngitis, Tracheitis and Laryngitis
Acute Pharyngitis and Tracheitis
Most sore throats are caused by viruses – including adenovirus and Epstein–Barr virus.
Bacteria include Streptococcus pyogenes, Haemophilus parainfluenzae and Corynbacterium diphtheriae.
Tonsillitis is a common acute inflammation, historically due to streptococcal infection but, with the introduction of antibiotics, viruses are the initiating infective agents in most cases.
Diphtheria, now uncommon in countries where vaccination is widespread, is a serious infection.
The formation of a pseudomembrane is striking.
This pseudomembrane may spread to block the larynx, causing respiratory obstruction.
The exotoxin of diphtheria is encoded by a bacteriophage (a virus which infects the bacterium). It can cause myocarditis (p.212) and neuropathy (p.569).
Acute laryngitis is often due to parainfluenza viruses. Acute oedema of the glottis is seen in some cases of anaphylaxis (p.101) and angioneurotic oedema.
Other Disorders of the Larynx
Chronic Laryngitis
Cigarette smoking, repeated attacks of infection and atmospheric pollution may lead to chronic inflammation of the larynx.
Two main features are (a) changes in the lining epithelium and (b) increase in mucus secretion.
The following sequence of events tends to take place:
Tuberculosis – this can cause severe ulceration of the larynx and is usually secondary to pulmonary tuberculosis.
Tumours of the Upper Respiratory Tract
Benign Tumours
Papillomas may be single but are often multiple and due to infection by papilloma virus. The epithelium may be one of two types:
Malignant Tumours
These are mainly squamous carcinomas and are commonly seen in the larynx. Intraepithelial neoplasia (carcinoma-in-situ) is a frequent precursor.
Smoking and alcohol consumption are aetiologically important. The rate of growth and spread is influenced by the site within the larynx.
Nasopharyngeal carcinoma is common in Eastern countries and is associated with Epstein–Barr virus infection.
Adenocarcinoma of the nose is a rare tumour, sometimes seen in woodworkers.
Lungs – Anatomy
Acinus
This is the functional unit of the lung, where gas transfer takes place. It consists of the respiratory bronchiole and associated alveolar ducts and sacs supplied by one terminal bronchiole. There are approximately 25 000 acini in the normal adult male lung.
Lobule
This is served by one preterminal bronchiole and is the smallest anatomic compartment of lung that is grossly apparent. It contains 3 to 30 acini and is bound by connective tissue septa. These connective tissue septa may be accentuated in smokers.
The total area for air exchange is very large (equivalent to a tennis court) allowing considerable reserve capacity.
Respiration
The normal intake of air is around 7 litres per minute; of this, after allowing for nonfunctioning dead space (trachea, bronchi, etc.), approximately 5 litres per minute are available for alveolar ventilation. A definite flow of air is maintained as far as the terminal bronchiole. Beyond this point the actual flow ceases and gas exchange is effected by diffusion.
Three factors are involved in the maintenance of adequate respiration:
Interference with any of these factors will result in respiratory embarrassment (dyspnoea) and even respiratory failure.
Inadequate Air Supply to Alveoli (Hypoventilation)
This may be due to lesions and diseases which interfere with the mechanics of respiration, such as central nervous lesions affecting the respiratory centre, paralysis of muscles of respiration as in poliomyelitis, injuries and deformities of the thoracic skeleton (e.g. fracture of ribs, kyphosis) and pleural disease preventing lung expansion as in pleural effusion or pneumothorax.
The most common cause of hypoventilation is bronchial obstruction. This may be reversible due to bronchial spasm as in asthma or irreversible in chronic obstructive pulmonary disease (p.258).
Impaired Diffusion of Gases
Three mechanisms may interfere with diffusion:
Altered Pulmonary Perfusion
Interference with the pulmonary circulation may occur in 4 main ways:
Note: In addition to hypoxaemia, inadequate perfusion tends to cause retention of carbon dioxide.
In chronic lung disease, ventilation, diffusion and perfusion disorders are present in varying degrees.
In many lung diseases this imbalance is increased. Admixture of well and poorly oxygenated blood results in hypoxaemia.
Acute Bronchitis
This is an inflammation of the large and medium bronchi. The condition may be serious if associated with pre-existing respiratory disease. Mucous and serous glands in the walls of the bronchi provide abundant mucoid secretion during the inflammation. Ciliated epithelia lining the bronchi aid passage of the exudate upward and help prevent spread down to the bronchioles.
In most cases, the process is initiated by a viral or mycoplasmal infection. It is a common complication of influenza and measles. This initial phase is followed by bacterial invasion. Streptococcus pneumoniae and Haemophilus influenzae are commonest, but Staphylococcus aureus and Streptococcus pyogenes may be found, especially in infants.
The condition is usually mild, and spread to the bronchioles is unusual in healthy adults due to the effective ciliary action of the bronchial epithelium. Spread may occur however in debilitated people. Bronchiolitis and bronchopneumonia result and can prove fatal. Equally important is the serious effect of repeated attacks of acute infection in patients with chronic bronchitis (p.258).
Bronchial Asthma
In asthma, there are spasmodic attacks of reversible bronchial obstruction with wheezing and dyspnoea and often a dry cough. The prevalence has markedly increased in recent years, although has now plateaued. There are 2 main patterns:
Other varieties include occupational asthma and aspirin-induced asthma.
The basic mechanism is as follows:
The histological changes are a combination of allergic reaction and muscular hypertrophy, the result of prolonged spasm.
Chronic Obstructive Pulmonary Disease (COPD)
This term describes three entities that show considerable clinical overlap:
COPD is common and is the fourth leading cause of death worldwide.
Aetiology The main factors are:
Chronic Bronchitis
The clinical definition is based on the presence of a productive cough lasting at least 3 months and occurring annually for at least 2 years.
Pathologically the following changes are seen:
The increase in thickness of the mucous gland layer is striking: at post-mortem the Reid index is measured – i.e. the ratio of the submucous layer (X) to the whole thickness (Y): a value greater than 1:2 is significant.
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