Introduction

This chapter concentrates on the physiology and pathology of the respiratory system, looking at the structure and function of the respiratory system in health.

The nose

In the breathing of a healthy person at rest, all air is drawn in through the nostrils into the nasal cavity. This space has a deeply ridged surface which has a rich blood supply. This means that the current of air comes into contact with a large area of warm epithelium before its descent towards the lungs. In addition to the cilia on the epithelial cells, body hairs are present in the opening of the nose to filter out any large foreign particles.

Opening out into the nasal cavity are the sinuses. These are cavities deep within the facial bones that have narrow passageways leading into the nasal cavity. Their function is to give the voice a resonant quality, the loss of which is apparent in anyone who has suffered from a ‘head cold’, when the sinuses become filled with thick secretions. A tiny duct leading from each eye also opens into this space. This continually drains away tears from the eyes. The runny nose that accompanies a bout of crying is a result of the excess tears flooding into the nose through this duct.

The nose is the specialised sense organ for smell. To perform this function, delicate nerves originating from the base of the brain penetrate the bone above the nasal cavity to supply the nasal lining. These are sensitive to the chemicals in the incoming air, which become dissolved in the mucus. The sense of smell is also important to enrich the experience of taste. Occasionally, in a head injury, these nerves become permanently severed, even though there may be no other major brain damage. The person may recover fully but be left without a sense of smell and with a highly impaired sense of taste.

Other nerves in the nose are very sensitive to the irritation that can result from foreign particles and from inflammation of the lining of the nose. These nerves can trigger the protective reflex of the sneeze, which is a very effective mechanism for expelling unwanted material from the nasal cavity.

The pharynx

The pharynx is the space at the back of the nose that connects with the throat. The adenoids (nasal tonsils) are two collections of lymph-node tissue situated at the back of this space. The tonsils (palatine tonsils), which are similar collections of lymphoid tissue, are situated a little lower down. The palatine tonsils can be seen in most people on either side of the space behind the arch of the palate, as illustrated in Figure 3.1a-II, whereas the adenoids are hidden up behind the palatoglossal arch.

Figure 3.3a-II • The location of the palatine tonsils.

The function of the tonsils is to complement the protective function of the lymphocytes in the respiratory epithelium. Their presence is an indication of the need for the lungs to be protected from foreign material. The adenoids are most active in early childhood, because this is the time when immunity to a wide range of infections has to be rapidly developed. After 5 years of the age, in most people the adenoids shrink to a negligible size, although the palatine tonsils persist into adulthood.

The eustachian tube opens from the middle ear into the pharynx close to the location of the adenoids. The middle ear is also lined with respiratory epithelium, and the eustachian tube allows the drainage of mucus from this enclosed space.

The larynx

The larynx, popularly known as the voice box, is the structure that separates the pharynx from the trachea. Although it is primarily associated with voice production, it also plays a very important protective role. Figure 3.3a-III is a diagrammatic representation of the nasal cavity, pharynx and larynx, which illustrates how these structures relate to each other.

Figure 3.3a-III • The pathway of the air from the nose to the larynx.

The tube-like larynx has stiff cartilage in its walls which enables it to maintain its shape. The cartilage at the front of the larynx is what can be seen and felt as the ‘Adam’s apple’. Because the thyroid gland sits close to this structure, the medical term for this is the ‘thyroid cartilage’.

The vocal cords are two fine flaps of tissue that are suspended from the surrounding cartilage tube. Figure 3.3a-IV shows a view of the aperture of the larynx. The vocal cords are depicted on either side of the aperture, leaving a triangular space through which the air can pass. Tiny muscles in the larynx contract to change the shape of this triangle between the vocal cords so that the space can vary from being a wide gap to a tiny ‘chink’. The tone of the muscles also affects the tightness of the vocal cords. During quiet breathing the gap is wide to allow a free passage of air in and out of the lungs. During speech, the gap narrows and the cords tighten. The expired air causes vibration of the tightened cords, which produces the sound of the voice. The tighter the cords, the higher the pitch of the voice.

Figure 3.3a-IV • The interior of the larynx, viewed from above.

The protective reflex of the cough involves a temporary tightening of the vocal cords so that the gap closes altogether. This occurs during a forced expiration of air. The closed gap means that the pressure of air below the vocal cords is suddenly increased. At this point the muscles relax and the gap widens. This results in a sudden forceful rush of air accompanied by the familiar sound of the cough. Like the sneeze, the cough is stimulated by the irritation of nerves in the larynx and the tubes that branch from the trachea into the lungs. The cough also has the effect of expelling unwanted material from the trachea, larynx and pharynx out through the open mouth.

The other important protective structure in the larynx is a stiff flap of cartilage-containing tissue called the ‘epiglottis’, which sits above the larynx. During swallowing this flap descends to completely cover the gap between the vocal cords. This protects against the inhalation of food during swallowing.

The trachea

The trachea is the wide tube that descends from the larynx to the division of the bronchi deep within the thorax. It is held open by rings of cartilage within its walls and is also lined by respiratory epithelium (see Q.3.3.a-1).

The bronchi and bronchioles

The bronchi and the smaller tubes into which they divide (the bronchioles) are structurally similar to the trachea, and are also lined with respiratory epithelium. Therefore, they too are concerned with ensuring the air that reaches the depths of the lungs is as warm, moist and as clean as possible.

The width of these air passages varies according to need. During exercise the bronchi and bronchioles widen, so that a large amount of air can enter the lungs in each breath. At rest, they narrow again because the need for a large volume of air is not so necessary, and because the protective mechanism of these tubes is more efficient when they are narrower.

The alveoli

At the end of each of the tiniest branches of the tree-like air passages that penetrate the lung are masses of tiny air sacs only a few cells in diameter. These are known as ‘alveoli’ (singular ‘alveolus’). The lungs are composed of millions of these tiny air spaces packed together, and all connecting with the outside world through the bronchioles and bronchi. The alveoli are lined with only a single layer of flat squamous epithelium, and are supplied by a dense network of fine blood capillaries.

Figure 3.3a-V illustrates how the alveoli branch from the bronchioles, and how a network of capillaries supplies each one.

Figure 3.3a-V • An alveolus and its capillary network.

The lungs

The lungs consist of the branching air passages of the bronchi and bronchioles, and the thousands of alveoli clustered around the ends of the bronchioles. The lungs are surrounded and supported by elastic connective tissue, which has the tendency to cause the fully expanded lung to contract. The lungs sit within the thorax, which is protected by the rib cage and its supportive muscles, and they rest on the sheet of muscle below known as the ‘diaphragm’.

The lungs are surrounded by a double layer of protective fluid-secreting serous membrane called the ‘pleura’. In the pleura the secreted fluid forms a thin lubricating layer between the two pleural membranes akin to the fluid that separates the pericardium from the heart. The outer pleural membrane is attached to the inner surface of the rib cage from above and at the sides, and to the diaphragm below. It is this attachment that prevents the elastic lungs from collapsing. Figure 3.3a-VI illustrates the relationship between the pleura and the lungs.

Figure 3.3a-VI • The relationship of the pleura with the lungs.

Introduction

The bulk of minor respiratory disease, particularly infectious disease affecting the upper respiratory tract, is managed in the UK entirely by the GP. More severe problems affecting the nose and throat may be referred for the opinion of a specialist in diseases of the ear, nose and throat.

A patient with any other severe respiratory disease will be referred by their GP to a specialist in respiratory medicine. These doctors are commonly known in the UK as chest physicians.

If a surgical operation is necessary, the chest physician will refer the patient to a thoracic surgeon. Very often the thoracic surgeon is also skilled in the surgery of the heart, in which case the proper title is ‘cardiothoracic surgeon’.

Physical examination

The physical examination of the respiratory system involves the stages listed in Table 3.3b-I.

Table 3.3b-I The stages of physical examination of the lungs

Laryngoscopy

This is an examination performed by an ear, nose and throat specialist to examine the vocal cords. It involves the skilled insertion of an instrument, which carries a tiny mirror, into the mouth and thence into the pharynx. Laryngoscopy is usually painless.

Examination of the sputum

A patient with infection or a possible tumour will be requested to produce a sample of phlegm. This can reveal the presence of infectious organisms, and occasionally in cancer can contain cancerous cells. In the case of infection, the sample can be used to test for sensitivity to a range of antibiotics.

Lung-function tests

Lung-function tests are performed when disease of the lower respiratory tract is suspected. These tests require the patient to blow into a machine that measures the force and volume of the out-breath, and detects the changes that are characteristic of the diseases which lead to restricted breathing, such as asthma and emphysema. These tests are straightforward and painless for the patient to perform.

The most simple of these tests is the peak expiratory outflow assessment, which is done using a peak-flow meter. This is a hand-held tool that the patient can be taught to use at home. Its main purpose is to assess the severity of asthma in a patient on ongoing treatment. It does not provide as detailed or helpful information as the more complex tests available in a hospital outpatient department.

Blood tests

Serum samples can be used to examine for antigens of infectious agents and for antibodies to confirm the presence of infection. Certain antibodies can also indicate the tendency to allergy, which is common in asthmatics.

In lower respiratory tract disease, a sample of arterial blood is drawn from the radial artery to measure the concentration of oxygen in the blood.

Chest X-ray imaging

This test is performed on all patients with suspected lower respiratory tract disease. Most serious disease, such as pneumonia, cancer, emphysema and lung fibrosis, will show characteristic changes on a chest X-ray image. However, in asthma, even when severe, the chest X-ray image usually appears normal.

Computed tomography (CT) scan

This test is most commonly used to assess the size and spread of lung cancer. CT will reveal not only the lung tissue, but also the lymph nodes deep within the chest.

Figure 3.3b-I shows a CT scan of the lungs in a patient with advanced lung cancer. This is a ‘slice’ of the upper chest presented as if the patient is lying on their back with their feet facing the viewer. The white structures are bones, and the sternum, one vertebral body, some ribs and the scapulae are visible. The very black areas are regions of normal air spaces in the lungs. The cancer is clearly seen as a grey shadow invading the normal lung tissue.

Figure 3.3b-I • A CT scan of the lung indicating a large area of cancerous tissue (arrow).

The less-invasive technique of magnetic resonance imaging (MRI) is less valuable in imaging of the tissues of the lung because the respiratory movements affect the quality of the images.

Bronchoscopy

This invasive test involves the passage of a flexible telescope into the wider air passages of a sedated patient. The tube can be inserted as far as the larger bronchioles. Tumours can be seen directly by means of this technique.

Alternatively, salt water can be squirted down the tip of the tube to retrieve cells from the deeper parts of the lungs in cases of possible infection or cancer when sputum does not reveal any features of disease. This technique is known as ‘bronchoalveolar lavage’.

Biopsy of the lung tissue

Biopsy involves the removal of a piece of lung tissue for examination under the microscope. The biopsy can be taken from the bronchi or bronchioles through the bronchoscope. Biopsy of the pleura can be taken through a small incision in the skin overlying a gap between the ribs. Sometimes a thoracic surgeon is required to open up the chest to remove tissue from deep in the chest.

Biopsy is most commonly used to confirm the diagnosis of cancer, but also can be used when infection is suspected, but cannot be detected by any other means.

Fluid can also be drawn from between the pleura by means of a syringe needle inserted between the ribs into the space. This is used only in those conditions that have caused fluid to accumulate in this space, known as pleural effusion. Such conditions include cancer of the bronchus, pneumonia and pulmonary oedema.

Introduction

Minor diseases of the upper parts of the respiratory tract such as experience of coughs, colds, and flu, are a well-recognised part of normal life. Conditions that affect the structures progressively further down the respiratory tract, such as the tonsils, larynx and trachea, are seen in conventional medicine as more serious conditions. Nevertheless, on the whole, most acute conditions of the upper half of the respiratory tract are relatively benign.

The conditions of the upper respiratory tract studied in this chapter are:

The common cold

The common cold virus affects the nose and pharynx (throat), leading to sore throat, stuffy and runny nose, and watery eyes. The secretions are usually clear. Fever is slight or absent. Simple treatments for the common cold include vitamin C and zinc preparations and steam inhalations to loosen secretions. Eucalyptus and menthol oils can be a helpful addition to a steam inhalation, and these aromatic compounds are also commonly found in proprietary cough sweets and medicines. Aspirin or paracetamol may be taken to relieve the discomfort of sore throat and to reduce fever.

Decongestant preparations that act by constriction of the blood vessels of the nose are not generally considered to be of great benefit. However, they are freely available as proprietary medications in the form of tablets or nasal sprays containing drugs similar in action to the chemical ephedrine. Decongestants reduce the swelling of the nasal lining and reduce the secretions. However, these are conventionally recognised to be suppressive, and it is recognised that the symptoms may recur in a more pronounced form when the drug wears off. Examples of decongestant medication include Vicks Sinex nasal spray and Sudafed nasal spray.

Some cold preparations available from the chemist contain mild sedatives, which help by allowing the person to rest.

A common cold is not seen to be serious in conventional medicine except in people who have a risk of further infection due to ongoing chronic disease or immunodeficiency.

Sinusitis

Sinusitis is the result of infection of the respiratory epithelium that lines the sinus cavities situated in the facial bones. Although it is commonly triggered by a virus infection, sinusitis can readily become more persistent due to subsequent bacterial infection. This is because the narrow opening of the inflamed sinuses into the nasal cavity easily becomes blocked, and the stagnation of the secretions within predispose to further infection, often by the ‘healthy’ bacteria that usually reside harmlessly in the nose.

Prolonged sinusitis is more likely to occur in certain susceptible people. Susceptibility can be the result of a congenital structural abnormality sinuses (and so could run in families), a tendency to produce excessive phlegmy secretions, and of smoking.

The symptoms of sinusitis include painful and tender sinuses, with foci of pain over the inner aspect of the eyebrows and in the centre of the cheekbones. In a persistent infection the patient may feel very unwell and feverish, and may have a headache over the forehead. Thick green secretions, indicating bacterial infection, may be produced.

Supportive measures include inhalation of aromatic oils, such as Olbas oil, in steam. Doctors will frequently prescribe antibiotics (most commonly amoxicillin) in a severe case of sinusitis, although increasingly other antibiotics such as doxycycline or erythromycin may have to be chosen because of increasing bacterial resistance to amoxicillin.

In susceptible people, sinusitis, even when treated with antibiotics, can become recurrent. Occasionally it can become chronic, with symptoms that never totally go away. In such cases the patient may be referred to an ear, nose and throat specialist for consideration of endoscopic sinus surgery, which is a means of widening and draining the sinus passages.

Tonsillitis and pharyngitis

It is very common for a virus infection to affect the back of the throat and/or the tonsils without also affecting the nasal lining. The inflammation that results from this sort of infection is termed ‘pharyngitis’ (sore throat) or ‘tonsillitis’, depending on the site of the worst symptoms.

Like the common cold, this sort of viral infection, although uncomfortable, should be mild and short lived. In more severe cases the tonsils can become very inflamed and enlarged, and this can be accompanied by high fever, headache, joint pains and malaise.

Infection with the bacterium Streptococcus pyogenes used to be a very common cause of tonsillitis and pharyngitis, although it is now less common. The bacterial tonsillitis is characteristically more severe than that resulting from a viral infection, and yellow areas of pus can often be seen on the tonsil. However, there is no absolute distinction, as some viral tonsillitis can be very severe, and may also generate pus on the tonsil. Bacterial tonsillitis and pharyngitis can be diagnosed by means of a throat swab, although this takes a few days to process.

The treatment for bacterial tonsillitis is penicillin(phenoxymethyl) rather than amoxicillin. Often, a doctor might choose to prescribe penicillin in a severe case of tonsillitis, even when it is uncertain that the cause is bacterial. The delays involved in processing a throat swab and the discomfort for the patient deter many doctors from performing this test. This is an example of ‘treating blind’ (see Chapter 2.4d). In many cases it may never be known whether the antibiotic helped because the tonsillitis naturally gets better in most people by 3–7 days of onset.

There are two reasons to prescribe antibiotics in tonsillitis. The first is that the symptoms of bacterial infection may be reduced in severity by penicillin. The second, more important, reason is that in some people the Streptococcus infection can have more serious complications. These are scarlet fever, rheumatic fever and glomerulonephritis (also known as acute nephritis). However, for reasons that are unclear, these complications are becoming increasingly uncommon, even when the use of antibiotics is accounted for. For this reason current medical guidelines (NICE 2008) recommend that antibiotics are not prescribed in uncomplicated cases of pharyngitis and tonsillitis.

In rare cases, one tonsil can become very inflamed and full of pus. This condition, known as ‘quinsy’ (peritonsillar abscess), is very debilitating, and is associated with fever. The treatment is with antibiotics and may require urgent surgical drainage of the pus from the tonsil. In some cases the enlarged tonsil may compromise breathing, in which case it becomes a surgical emergency.

In some people, and children in particular, tonsillitis can become recurrent. It used to be common practice to remove the tonsils surgically in such cases. However, surgery is now performed only relatively rarely, as it is recognised to be a risky and traumatic experience for a child, and does not necessarily reduce recurrent attacks of pharyngitis. Tonsillectomy would only currently be considered if attacks of tonsillitis exceeded five a year for at least 2 years.

Glandular fever (infectious mononucleosis)

Glandular fever is one of the more severe forms of viral tonsillitis, and is caused by a virus known as the Epstein–Barr virus (EBV). However, most cases of EBV infection are mild. The evidence for this comes from studies of healthy students, in which blood tests showed that up to 90% of the students had contracted the infection at some point in their lives. Most of these infections had occurred without the students knowing that they had contracted glandular fever.

However, severe cases can be prolonged and draining. The patient can suffer from severe tonsillitis, with fever and joint aches. The lymph nodes in the neck are enlarged. There may be abdominal pain as other lymph tissue in the body, including the spleen, can become enlarged, and the liver may become inflamed. Occasionally, a red, measles-like rash can develop, particularly in patients who have been mistakenly prescribed amoxicillin because a bacterial tonsillitis was suspected. In very rare cases the virus can affect the lining of the brain and cause meningitis.

EBV infection can be diagnosed by a blood test that detects antibodies to the virus in or after the second week of symptoms. Treatment is supportive only. In severe cases, when there is enlargement of the spleen, corticosteroid tablets may be prescribed.

Laryngitis, tracheitis and croup

Some of the other viral infections primarily affect the larynx and the trachea. The result is inflammation of these tissues, leading to one or more of hoarseness, cough and pain felt beneath the sternum. The pain is related to coughing and deep breathing.

Again, these infections may only be short lived except in those who are susceptible, particularly young children and smokers. In susceptible people there is a risk that tracheitis can descend to involve the bronchi and cause a much more severe and prolonged infection.

In children under the age of 3 years the swelling of the vocal cords can be so severe as to lead to difficulty in breathing. The affected child will be hoarse, have a rapid respiratory rate and may have a barking cough. If severe, a rasping noise, known as ‘stridor’, is heard as the child breathes in. In very severe cases the child needs to be nursed in a steam-filled tent in an intensive care unit to allow for safe recovery. The barking cough in such cases is called ‘croup’, and may persist in a child for weeks.

In some cases laryngitis may persist to give a syndrome of persistently hoarse voice and thick stringy mucus that is difficult to clear. This is known as ‘chronic laryngitis’. The patient is otherwise well. This is more common in people who smoke or in those who overuse their voices, such as singers.

The treatment of chronic laryngitis is to rest the voice and avoid smoking. Steam inhalations can soothe the vocal cords and loosen the mucus. However, if the hoarseness persists for more than 3 weeks it is considered good practice for the patient to be referred to an ear, nose and throat specialist. The vocal cords can then be examined by laryngoscopy to exclude the unlikely possibility of a tumour of the vocal cords.

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