Eureka: Respiratory Medicine

Chapter 1

First principles

Overview of the respiratory system

Development of the respiratory system

Anatomy of the respiratory tract

Ventilation and respiration

Control of ventilation

Acid–base balance

Pulmonary circulation

Lung immunity and inflammation

Overview of the respiratory system

Starter questions

Answers to the following questions are on page 57.

1.   Does the respiratory system have a role in more than breathing?

2.   Do humans have an efficient respiratory system?

The respiratory system comprises the nose, the mouth, the pharynx, the larynx, the trachea and the bronchial tree and alveoli (Figure 1.1). The system also includes the pleurae (the two membranes covering the whole of the lungs) and the muscles and bones of the chest wall and diaphragm.

Roles of the respiratory system

The main role of the respiratory system is to supply the blood with oxygen. Oxygenated blood is then distributed throughout the body by the circulatory system (the heart and blood vessels). Oxygen is essential for the generation of energy for cellular processes.

The respiratory system also excretes carbon dioxide, which is a waste product of cellular reactions. Carbon dioxide diffuses from cells into the blood, which is then transported to the lungs by the circulatory system. Failure to excrete carbon dioxide from the lungs would result in an increased amount of carbon dioxide in the body. The increased carbon dioxide would, in turn, increase the amount of acid in the body and have other toxic effects.

More severe respiratory conditions prevent the supply of oxygen to the blood, result in failure to excrete carbon dioxide, or both. The consequences are life-threatening.

The respiratory system also regulates temperature, defends against pathogens and creates the sounds that allow us to talk.

Structure and function of the respiratory system

Oxygen uptake by the lungs begins with inhalation of air through the mouth or nose. This process uses the muscle power of the diaphragm and chest wall. From the mouth or nose, inhaled air flows through the pharynx and larynx. The air travels into the lungs through the bronchial tree (Figure 1.2).

Figure 1.1 Structure of the respiratory system.

The bronchial tree starts with the trachea, which splits into two smaller tubes: the right main bronchus and the left main bronchus. The right and left main bronchi supply air to their respective lungs. Both main bronchi divide further into smaller and smaller tubes. These tubes eventually supply air to small air sacs called alveoli.

Oxygen in the air diffuses into the blood carried by capillaries (small blood vessels) surrounding the alveoli. At the same time, carbon dioxide passes from the blood in the capillaries to the alveoli. The carbon dioxide is then removed from the body during expiration. This process expels air from the lungs, following the same path as inhalation but in reverse.

Figure 1.2 The brochial tree. As each bronchus divides into smaller bronchioles, less and less cartilage surrounds the airway.

Development of the respiratory system

Starter questions

Answers to the following questions are on page 57.

3.   Do fetuses breathe in the womb?

4.   Can babies survive if their respiratory system does not develop normally?

The development of the respiratory system is complicated and mostly happens in utero and postnatally, although the respiratory system continues to develop into adolescence.

Fetal development

The respiratory system develops from the midsection of the foregut, just anterior to the pharynx, in the 4th week of gestation (about 22 days after implantation) (Figure 1.3). First, a laryngeotracheal groove forms on the ventral side of the foregut. The groove deepens to become a pouch called the respiratory diverticulum. The diverticulum separates from the oesophagus, then splits into a right and a left bronchial bud around day 26.

Over the next 2 weeks, secondary bronchi are produced by further branching. By week 5, asymmetrical branching has occurred and the lung lobes have formed. By week 6, the main divisions of the bronchial tree are in place.

The distal end of the respiratory diverticulum develops into the tracheal bud. This structure gives rise to the trachea.

All cells derive from one of the three embryonic germ cell layers: the endoderm (inner layer), the mesoderm (middle layer) or the ectoderm (outer layer). The respiratory diverticulum is lined by endoderm. Therefore all the respiratory epithelium and the glands of the trachea, bronchi and alveoli are endodermal in origin. Supporting structures, such as cartilage, blood vessels, muscles and connective tissue, derive from the mesoderm surrounding the respiratory diverticulum. The mesoderm regulates the way in which branching occurs; mesoderm around the trachea inhibits branching, and mesoderm around the bronchi stimulates it.

The further development of the respiratory system is divided into three stages: the glandular period, the canalicular period and the terminal saccular period. Development is regulated by several factors. These factors and their functions are shown in Table 1.1.

Figure 1.3 Development of the respiratory system.

Glandular period

In the glandular period (weeks 7–16), all the major lung elements develop. The bronchial tree repeatedly branches to the level of the terminal bronchioles. The glandular period is so-called because the terminal bronchioles resemble small clusters of gland cells called glandular acini.

Canalicular period

In the canalicular period (weeks 16–26), the bronchioles, alveolar ducts (connecting the bronchioles to the sacs containing the alveoli) and primitive alveoli develop. The lung tissue becomes very vascular and capillaries develop.

Terminal saccular period

In the terminal saccular period, weeks 26–40, more alveoli develop and mature. Alveolar cells differentiate into type 1 and type 2 pneumocytes.

Type 1 pneumocytes predominate and are the specialised cells responsible for gas exchange.

Type 2 pneumocytes secrete the pulmonary surfactant that decreases the surface tension of the mucoid lining of the alveoli.

Also in this period, the epithelium thins as it develops and surfactant secretion begins. Capillaries continue to develop around the alveoli.

Surfactant reduces the effort needed to expand the lungs by decreasing surface tension and increasing pulmonary compliance (expansion in response to transmural pressure). Because surfactant secretion does not start until weeks 26–40 some premature babies develop infant respiratory distress syndrome.

Development of pleural cavities and diaphragm

The major body cavities originate from spaces in the lateral plate mesoderm and the cardiogenic mesoderm. The two spaces fuse to form a horseshoe-shaped cavity. Part of this becomes the pericardial cavity; the other part becomes the peritoneal and pleural cavities. As the lungs develop, pleural sacs form around them. The diaphragm develops from the septum transversum, pleuroperitoneal membranes, dorsal mesentery of the oesophagus and body cavity walls.

The fetus aspirates fluid into the lungs throughout development. This fluid contains surfactant, mucus and amniotic fluid. At birth, the fluid is rapidly reabsorbed into blood and lymph vessels.

Postnatal development

The lungs of a newborn baby contain about 30 million alveoli. Alveoli multiply rapidly in the first 2–3 years of life. Little multiplication occurs subsequently, and an adult’s lungs contain about 500 million.

The number of alveoli plateaus in early childhood. However, their size and surface area increase into adolescence.

The lungs provide a huge surface area for gas exchange. They contain 2400 km of airways, about the distance from London to Istanbul. The surface area of the 500 million alveoli within is around 100 m², the same surface area as a tennis court.

Anatomy of the respiratory tract

Starter questions

Answers to the following questions are on page 57–58.

5.   Why is the nose important for breathing?

6.   Why do we need bronchioles and alveoli?

7.   Why are the lungs of an athlete bigger than those of a non-athlete?

The whole airway extends from the nostrils and lips to the alveoli in the lungs. At the level of the vocal cords, the respiratory tract splits into the upper airway and the lower airway. Any part of this system can be affected by respiratory disease.

Upper airway

The upper airway comprises the nose and nasal cavity, the mouth and oral cavity, and the pharynx and larynx. Its overall function is to provide a passage for air to be inhaled and exhaled.

Nose

The nose:

moistens, warms and filters air

provides the sense of smell

The external part of the nose consists mostly of cartilage but also of bone (frontal, nasal and maxillary) (Figure 1.4). Bones form part of the nasal bridge superiorly. This part of the nose is covered by skin and muscle, and is lined by a mucous membrane and coarse hairs. It contains sweat glands and sebaceous glands.

The nasal airway extends from the nostrils (anterior nares) on the face to the choanae (posterior nares) at the pharynx. Two nasal vestibules lead to a common nasal cavity.

The nasal cavity extends from the vestibule horizontally and is lined by ciliated columnar epithelium. It has an arched ceiling that extends upwards to the olfactory area. This region is made of olfactory epithelium over the cribriform plate, and is supported by nasal, frontal, ethmoid and sphenoid bones.

The medial wall of the nose, the nasal septum, separates the two nasal cavities. The septum is formed by the ethmoid bone and the vomer. The inferior aspect of the nose comprises the palatine process of the maxilla, the palatine bone and the soft palate. The lateral wall of the nasal cavity consists of maxillary and ethmoid bones.

Figure 1.4 Anatomy of the nose.

Inflammation of the nasal passages (rhinitis) or sinuses (sinusitis) result in airflow obstruction, excess mucus secretion and sinus pain. It is caused by infectious, allergic or non-allergic conditions that are frequently associated with lung disease, e.g. allergic rhinitis (one component of hay fever) is associated with asthma, and viral rhinitis with acute bronchitis.

Three bony folds (the superior, middle and inferior turbinates) run horizontally and increase the surface area of the nose.

The nasolacrimal duct runs into the inferior meatus below the inferior nasal cavity

The frontal, anterior, ethmoidal and maxillary sinuses drain into the middle meatus

The posterior ethmoidal sinus drains into the superior meatus

The sinuses are lined by mucus-secreting respiratory epithelium.

The sphenoethmoidal recess is a small area of the nose that sits above the superior turbinates in front of the sphenoid bone. The sphenoidal air sinus opens into this area.

Blood supply

The nose is supplied by the ophthalmic, maxillary and facial arteries. Venous drainage is to the facial vein and pterygoid venous plexus.

Nerve supply

The olfactory mucosa is supplied by the olfactory (first cranial) nerve. Olfactory nerve fibres arise from olfactory cells in the olfactory mucous membrane. The fibres reach the olfactory bulbs through the cribiform plate.

The rest of the nasal cavity is supplied by the first and second divisions of the trigeminal nerve. The anterior part of the nose is supplied by the anterior ethmoidal nerve and the posterior part from the pterygopaltine ganglion.

Anosmia, lack of the sense of smell, has numerous causes, which affect any part of the pathway from the olfactory mucosa to the brain. Congenital causes include primary ciliary dyskinesia. Acquired causes include nasal polyps, snorting cocaine, tumours (e.g. suprasellar meningioma), trauma to the cribriform plate, inflammation (e.g. granulomatosis with polyangiitis) and brain lesions (e.g. cerebrovascular accident).

Lymphatics

The lymphatics of the vestibules drain into the submandibular nodes. The rest of the lymphatic drainage of the nose is to the upper deep cervical nodes.

Cancer cells and the inflammatory response to infection spread along the route of local lymph drainage. Therefore knowing the lymphatic drainage for a particular organ is important for identifying potential sites of lymphadenopathy caused by metastases and infection.

Histology

Excluding the vestibules, the nasal cavity is lined by mucous membrane. The olfactory mucous membrane covers the upper part of the superior turbinates and sphenoethmoidal recess. It also lines the roof of the nasal septum. This area contains olfactory neurone and is therefore essential for sensing smell.

The lower part of the nasal cavity is lined by respiratory mucous membrane. The role of the respiratory mucous membrane is to warm, moisten and clean inspired air. The air is warmed by a plexus of veins in the submucous connective tissue. Moisture comes from mucus secreted by glands and goblet cells. The sticky surface of the mucous membrane helps remove dust from the air. The ciliary action of the columnar ciliated epithelia on the surface moves the mucus backwards towards the pharynx, where it is swallowed.

Pharynx

The pharynx is behind the nasal cavities, mouth and larynx (Figure 1.5). It has a musculomembranous wall, except anteriorly, where the posterior nasal apertures, opening of the mouth and inlet of the larynx sit.

Nasopharynx

This portion of the pharynx is between the nasal cavities and soft palate. The nasopharynx has a roof and a floor, as well as anterior, posterior and lateral walls. The sphenoid and occipital bones support the roof. The floor is formed by the soft palate, the anterior wall by the posterior nasal apertures, and the posterior wall by the anterior arch of the atlas. The lateral walls contain the opening of the auditory tubes.

Oropharynx

The oropharynx extends from behind the mouth to the soft palate and upper border of the oesophagus. Its roof is formed by the underside of the soft palate and pharyngeal isthmus. The floor is formed by the posterior third of the tongue and the space between the tongue and the epiglottis.

The anterior wall opens into the mouth through the oropharyngeal isthmus.

Figure1.5 Anatomy of the pharynx.

The posterior wall is supported by the 2nd and 3rd cervical vertebrae.

The lateral walls have palatoglossal and palatopharyngeal arches, folds of mucous membrane covering the palatoglossus muscle.

Snoring is the coarse sound produced by vibration of the soft palate and other tissue in the upper airway. It results from partial blockage of airflow in the nose, oropharynx or base of the tongue. This blockage can have serious physiological consequences in a disease called obstructive sleep apnoea.

Hypopharynx

The hypopharynx sits behind the opening of the larynx and posterior laryngeal surface. It runs from the upper border of the epiglottis to the lower border of cricoid cartilage. The anterior wall is formed by the inlet and posterior surface of the larynx. The posterior wall is supported by the 3rd to 6th vertebrae, and the lateral wall by the thyroid cartilage and thyrohyoid membrane.

Blood supply

Arterial supply to the pharynx is from branches of the ascending pharyngeal, ascending palatine, facial, maxillary and lingual arteries. Veins drain into the pharyngeal venous plexus, which drains into the internal jugular vein.

Nerve supply

Nerve supply is from the pharyngeal plexus. The plexus consists of branches of glossopharyngeal, vagus and sympathetic nerves.

Lymphatics

The tonsils sit in the roof of the pharynx. Lymph vessels drain into deep cervical, retropharyngeal or paratracheal nodes.

Histology

The pharyngeal wall has three layers: a mucous membrane, a fibrous layer and a muscle layer.

The first layer, the mucous membrane, is continuous with the mouth, nasal cavities and larynx. It is also continuous with the tympanic cavity, through the auditory tubes. The upper part of the membrane is lined by ciliated columnar epithelium, and the lower part with stratified squamous epithelium.

The middle layer of the pharyngeal wall is a fibrous layer. It is thick at the top, where it connects with the base of the skull, and continuous with the submucosa of the oesophagus.

The third layer is a muscle layer. It consists of the superior, middle and inferior constrictor muscles, as well as the stylopharyngeus and salpingopharyngeus muscles.

Larynx

The larynx is:

a conduit for the passage of air

a sphincter that closes during swallowing to prevent food from entering the respiratory tract

the organ of phonation (including speech).

The larynx is also essential for effective coughing and for Valsalva’s manoeuvre.

The larynx sits between the trachea and the pharynx (Figure 1.6). It is anterior to the oesophagus, at the level of the 3rd to 6th cervical vertebrae, and extends from the epiglottis to the cricoid cartilage.

The thyrohyoid membrane suspends the larynx from the hyoid bone. This membrane is made of three single cartilages (thyroid, epiglottic and cricoid) and three paired cartilages (arytenoid, corniculate and cuneiform), combined with ligaments, membranes and muscles.

The main body of the larynx is formed by the thyroid cartilage (Adam’s apple). The Adam’s apple is connected superiorly to the thyrohyoid membrane. It is formed by two laminae that join anteriorly at the midline. The posterior aspects of the laminae have superior and inferior horns. These horns articulate inferiorly with the cricoid cartilage.

Figure 1.6 Anatomy of the larynx. (a) Anterior view. (b) Posterior view.

The epiglottic cartilage arises from the posterior surface of the thyroid cartilage. During breathing, the leaf-shaped epiglottis remains in an upward position, allowing air to pass into the trachea. During swallowing, the epiglottis projects superiorly to cover the laryngeal inlet, diverting food to the oesophagus and away from the trachea. Thus the epiglottis prevents aspiration of food.

The male larynx and female larynx are similar in size during childhood. However, at puberty the male larynx enlarges significantly, reaching an average size in men of 45 mm long and 35 mm in diameter. The average size of the larynx in women is 35 mm by 25 mm.

The cricoid cartilage forms the only complete ring around the larynx, providing it with a rigid structure. Posteriorly, the cricoid cartilage is connected to the trachea by the cricotracheal ligament. Superiorly, it is connected to the thyroid and arytenoid cartilages by the cricothyroid membrane.

The arytenoid cartilages are two pyramidal structures that articulate with the posterior border of the cricoid. Their anterior projections are the vocal processes, which form the posterior attachment for the vocal cords.

The corniculate and cuneiform cartilages are small protrusions above the arytenoid cartilages.

Vocal cords

These are attached to the vocal processes posteriorly and the back of the thyroid cartilage anteriorly (Figure 1.7).

The vocal cords are tensed by the cricothyroid muscle, which pulls the arytenoid cartilages posteriorly. Conversely, the thyroarytenoid muscle relaxes the cords.

The posterior cricoarytenoid muscle externally rotates the arytenoid cartilages to induce vocal cord abduction. In contrast, the lateral and interarytenoid muscles cause adduction by internal rotation.

Figure 1.7 Coronal view of the vocal cords.

Blood supply

The superior and inferior laryngeal arteries derive from the superior and inferior thyroid arteries, respectively. Venous drainage occurs into the internal jugular and brachiocephalic veins through the superior and inferior thyroid veins.

Nerve supply

The vagus nerve innervates the larynx through the superior and recurrent laryngeal nerves. Sensory supply is provided by the internal branch of the superior laryngeal nerve above the vocal cords, and the recurrent laryngeal nerve below.

The external branch of the superior laryngeal nerve provides motor supply to the cricothyroid muscle. All other intrinsic muscles are innervated by the recurrent laryngeal nerve.

Paralysis of the left vocal cord is occasionally a sign of lung cancer. The left recurrent laryngeal nerve has an unusual course, running through the mediastinum and around the left main bronchus before reaching the larynx. Therefore it can be paralysed by left-sided hilar tumours or enlarged lymph nodes.

Lymphatics

The lower and upper deep cervical lymph nodes receive lymphatic drainage from below and above the vocal cords, respectively.

Histology

Above the vocal cords, the larynx is lined by stratified squamous non-keratinised epithelium. Below the vocal cords, it is lined by ciliated columnar pseudostratified epithelium containing numerous goblet cells (Figure 1.8).

Trachea

The trachea (Figure 1.2) is a mobile tube that is both cartilaginous and membranous. It starts at the lower border of the cricoid cartilages of the larynx. It then runs inferiorly along the midline of the neck. The trachea ends by dividing into the two main bronchi in the thorax at the level of the 4th and 5th thoracic vertebrae.

Figure 1.8 Epithelial layers of the larynx. (a) Non-keratinised stratified squamous epithelium. (b) Ciliated columnar pseudostratified epithelium.

The trachea is flattened posteriorly, so it is D-shaped in cross-section. It is about 11 cm long and 2–2.5 cm in diameter.

In the neck, skin, fascia, isthmus of the thyroid, inferior thyroid veins, jugular arch, sternothyroid muscles and sternohyoid muscles run in front of the trachea. The right and left recurrent laryngeal nerves, oesophagus and vertebrae run behind it. Lateral to the trachea are lobes of the thyroid gland and carotid sheath. In the thorax, the manubrium sterni, thymus, left innominate vein, aortic arch, innominate and left common carotid arteries, and deep cardiac plexus sit in front of the trachea.

In the thorax, the trachea sits in the superior mediastinum. On the right side, the trachea is in contact with the pleura, right vagus nerve and innominate artery. On the left side, it is in contact with the left recurrent nerve, aortic arch, and left common carotid and subclavian veins.

The trachea runs just in front of the oesophagus. Oesophageal tumours can erode into the trachea to cause oesophagotracheal fistulas. Tracheal tumours sometimes block the oesophagus to cause dysphagia (difficulty swallowing).

Blood supply

In the neck, the inferior thyroid arteries supply the trachea. In the chest, it is supplied by the bronchial arteries. The bronchial arteries form anastomotic networks with branches of the inferior thyroid artery.

Nerve supply

The trachea is innervated by the vagus nerve, recurrent laryngeal nerves and sympathetic trunks. Sympathetic fibres from sympathetic trunks supply smooth muscle and blood vessels.

Lymphatics

In the neck, lymphatic vessels drain into pretracheal and paratracheal lymph nodes. In the chest, drainage is into tracheobronchial lymph nodes. Paratracheal lymph nodes drain into the thoracic and mediastinal thoracic ducts.

Histology

The trachea is lined by a mucous membrane that is continuous with the larynx and bronchi. The membrane consists of a basement membrane and a stratified epithelium. The top layer is made of ciliated columnar epithelium. Below the basement membrane are elastic fibres and a submucosal layer containing connective tissue, blood vessels, mucous glands and nerves.

Lower airway

The lower airway comprises the trachea, bronchi, bronchioles and alveoli. Its main function is gas exchange.

Lungs

The lungs sit on either side of the mediastinum and fill most of the thorax. These two conical organs are separated by the heart, great vessels and other mediastinal structures. They comprise the bronchial tree and alveoli, and are covered by a thin membrane, the visceral pleura.

The lungs are soft, sponge-like and elastic. In children, they are pink. However, they darken with age because of the accumulation of inhaled dust particles.

To accommodate the heart, the left lung is about 10% smaller than the right lung. An oblique fissure divides the left lung into upper and lower lobes. An oblique and a horizontal fissure divide the right lung into upper, middle and lower lobes. Each lobe is further subdivided into segments (Figure 1.9).

Each lobe is supplied by a separate lobar bronchus. Each bronchus subdivides into segmental bronchi. Each segmental bronchus is part of a bronchopulmonary segment, which has its own segmental artery, lymph nodes and nerves.

In situs inversus, the major organs in the thorax and abdomen are on the opposite side to normal. When the heart is on the right (dextrocardia), the left lung has three lobes and the right lung has two. This congenital condition usually has no adverse effects for the patient. However, it can confuse physicians during clinical examinations. Situs inversus sometimes coexists with ciliary dysfunction. In such cases, it is associated with bronchiectasis and called Kartagener’s syndrome.

The mediastinal (medial) surface of each lung contains a hilum. This is where bronchi, lymphatic vessels, nerves and pulmonary blood vessels enter and leave the lung. These structures are held in place by connective tissue and pleura.

Bronchial tree

The trachea branches into the right and left main bronchi; these are the primary bronchi (Figure 1.2). The division point is called the carina. The right bronchus is shorter, more vertical and wider than the left.

The walls of the primary bronchi contain incomplete cartilaginous rings, like the trachea, and divide into secondary bronchi as they enter the lungs. Cartilage is present until the small bronchi. The right main bronchus divides into superior, middle and inferior lobar bronchi. The left divides into superior and inferior lobar divisions, corresponding to the lung lobar divisions.

Figure 1.9 Bronchopulmonary segments of the lung. (a) Lateral view. (b) Medial view. (1) Oblique fissures. (2) Horizontal fissure.

Secondary bronchi branch into tertiary bronchi. These bronchi, in turn, branch into bronchioles. Further branching eventually produces terminal bronchioles. From the trachea to the terminal bronchioles there are about 25 orders of branching. The bronchi and bronchioles originating from the branching of the trachea make up the bronchial tree.

Blood supply

Blood is supplied to the bronchial tree by bronchial arteries. The bronchial arteries arise from the descending aorta (Table 1.2). The bronchial veins drain into the azygous and hemiazygous veins (Table 1.3).

Nerve supply

The bronchial tree is innervated by the pulmonary plexus, which is formed by branches of the vagus and sympathetic trunk (Table 1.4).

The release of adrenaline (epinephrine) and noradrenaline (norepinephrine) relaxes smooth muscle in the bronchioles, which dilates the airway. Parasympathetic mediators have the opposite effect.

The right main bronchus is wider and more vertical than the left. This arrangement means that inhaled foreign bodies are more likely to enter the right main bronchus.

Lymphatics

Drainage is into the superficial and deep plexuses. These groups of lymphatic capillaries converge to form large lymphatic vessels and drain into bronchial lymph nodes.

Histology

The bronchial tree is lined by pseudostratified ciliated columnar epithelium (Figure 1.8b). Bronchioles also contain some non-ciliated columnar cells called club (Clara) cells. Club cells produce surfactant protein and can also act as stem cells. Stem cells have the potential to develop into various types of epithelial cell (Figure 1.8).

Alveoli and interstitium

The terminal bronchioles at the end of the bronchial tree open into small cavities in the lung parenchyma. These cavities are the alveoli (Figure 1.10).

Each terminal bronchiole supplies several alveoli, which are lined by respiratory epithelium. Alveoli are closely associated with an extensive network of capillaries, which cover about 70% of their surface.

Each lung of an adult human contains about 500 million alveoli. These provide a massive surface area for gas exchange: about 75 m² in an adult.

Blood supply

The lungs have a dual circulation.

The bronchial circulation supplies oxygenated blood for metabolic activity by the bronchial tree, lymphatics, visceral pleura and pulmonary vessels

Figure1.10 Alveoli receive deoxygenated blood from the pulmonary arterioles. Oxygenated blood leaves the alveoli through the pulmonary venules.

The pulmonary circulation (Figure 1.11) is a low-pressure system. It supplies the alveoli and interstitium with deoxygenated blood for oxygenation

The pulmonary artery arises from the right ventricle. It then divides into right and left pulmonary arteries, which follow the right and left main bronchi, respectively, to enter the lung at the hilum.

As the pulmonary arteries follow the bronchi, they divide further in the same pattern as the bronchial tree. At the terminal bronchioles, pulmonary arterioles supply the capillary plexus, which covers the alveoli, with deoxygenated blood.

Oxygen diffuses from the alveoli into the blood. Carbon dioxide diffuses from the blood into the alveoli.

Oxygenated blood from the alveolar capillaries drains into pulmonary venules. The venules merge to form pulmonary veins that accompany the bronchial tree. The veins leave the lungs at the hilum as right and left inferior and superior pulmonary veins. Each of these veins drains directly into the left atrium.

Figure 1.11 The pulmonary circulation. Blood supply to the alveoli is separate from that to the rest of the body. The pulmonary artery carries deoxygenated blood from the right ventricle to the lungs. Oxygenated blood returns to the left atrium through the pulmonary vein.

Major haemoptysis is the coughing up of large amounts of blood. The condition is often caused by bleeding from a bronchial artery that has enlarged in response to lung disease, rather than bleeding from the low-pressure pulmonary circulation.

The pulmonary artery and pulmonary vein are unique. The pulmonary artery is the only artery in the body to carry deoxygenated blood. The pulmonary vein is the only vein to carry oxygenated blood.

Anatomically, there is some overlap between the bronchial and pulmonary circulation; there are regions of the lung that both circulations supply. However, they differ physiologically. The arterial side of the bronchial circulation carries oxygenated blood at the same pressure as the rest of the systemic circulation. In contrast, the pulmonary artery carries deoxygenated blood at about a quarter of the pressure of the systemic circulation.

Deoxygenated blood in the bronchial circulation of the smaller bronchi drains to the pulmonary venous system. Thus the system is a minor arteriovenous shunt (a small hole between the arterial and venous systems). Bronchial veins supplying the larger airways drain into the azygous (right side) or hemiazygous (left side) veins.

Nerve supply

At each hilum is a pulmonary plexus with efferent and afferent autonomic nerves (Table 1.4). The plexus comprises branches of the vagus nerve and sympathetic trunk. The nerve supply runs down the bronchial tree, with little input to the alveoli. The walls of the alveoli contain stretch receptors, which are mechanoreceptors that respond when the lungs expand.

Lymphatics

Fluid in the air spaces of the alveoli is absorbed through the alveolar walls into the interstitium. From here, fluid travels along the bronchioles to lymph vessels (Table 1.5). These vessels merge as they follow the line of the bronchial tree to the hilum. They go through successive collections of lymph nodes before draining into the thoracic duct.

Histology

The walls of alveoli consist of a single-cell layer of squamous epithelium over a thin elastic basement (Figure 1.12). They are supported by extracellular matrix and capillaries. Alveolar walls contain holes that connect alveoli and terminal air ducts; these are the pores of Kohn.

The luminal surfaces of alveoli and the bronchial tree contain alveolar macrophages. When two or more alveoli have a common entrance, they are called an alveolar sac.

From the alveolar air space to the blood plasma are four layers. These layers are the respiratory membrane, across which oxygen must diffuse.

Type 1 and type 2 pneumocytes (alveolar wall)

Epithelial cell basement membrane

Capillary basement membrane

Capillary endothelium.

The respiratory membrane is 0.5 μm thick.

The interstitium surrounding alveoli is sparse to allow efficient gas exchange. However, it does contain collagen and elastin fibres. Expansion of the interstitial space is the major pathological change in interstitial lung diseases.

Alveolar epithelium consists of two types of cell: type 1 and type 2 pneumocytes.

Type 1 pneumocytes (squamous alveolar cells) are the main structural cells of the alveoli. They form a simple squamous epipthelial layer that covers 90% of the alveolar surface. Their large surface area and thin cytoplasmic layer allow rapid diffusion of oxygen into the blood

Type 2 pneumocytes are ciliated, more cuboid cells. They tend to occupy the corners of alveoli and secrete surfactant. Their surface contains microvilli, which increase the surface area of the cell.

Figure 1.12 The alveolar epithelium.

Some diseases reduce movement of gases across the respiratory membrane.

Idiopathic pulmonary fibrosis and sarcoidosis scar the lung parenchyma and thicken the interstitium

Pulmonary oedema in heart failure, and inflammation in hypersensitivity pneumonitis, cause swelling of the interstitium.

Thoracic wall

The thoracic wall is the outer margin of the thorax. The skin and muscle of the thoracic wall attach it to the shoulder girdle and trunk. The bony parts of the thoracic wall form the thoracic cage (Figure 1.13).

The thoracic wall consists of:

the thoracic part of the vertebral column posteriorly

the sternum and costal cartilages anteriorly

the ribs and intercostal spaces laterally

the suprapleural extension superiorly

the diaphragm inferiorly

The thoracic cavity is lined by the parietal pleura.

Thoracic vertebrae

The thoracic spine has an anterior concave curve. It comprises 12 thoracic vertebrae and intervertebral discs (Figure 1.14a). Each vertebra has a heart-shaped body from which a long spinous process descends.

Either side of the vertebral body are the costal facets. The superior and inferior costal facets are the sites at which each vertebra forms joints with the head of a rib. At the transverse costal facet, the transverse process articulates with the tubercle of a rib (Figure 1.14b and c).

Ribs

Ribs are long, curved, flat bones with a smooth upper border and a sharp thin inferior border (Figure 1.14c). The costal groove sits in the inferior border of the rib and contains the intercostal vessels and nerve (Figure 1.15). Each rib has a head, neck, tubercle, shaft and angle.

The 12 pairs of ribs are attached posteriorly to the thoracic vertebrae. Hyaline cartilages called costal cartilages connect the upper seven ribs to the lateral aspects of the sternum, and the 8th to 10th ribs to the cartilage immediately above. The costal cartilages of the 11th and 12th ribs are freestanding, with no anterior attachment. Costal cartilages provide the thoracic cage with its elasticity and mobility.

Figure 1.13 The thoracic cage.

Figure 1.14 The thoracic vertebrae. (a) The 12 thoracic vertebrae. (b) Lateral view. (c) Axial view.

Figure 1.15 The intercostal space. The vein, artery and nerves make up the neurovascular bundle.

The vertebrae and ribs are common sites of disease that cause chest pain. These diseases include vertebral collapse resulting from osteoporosis, spinal tuberculosis and metastases from lung or other cancers.

Sternum

The sternum is a flat bone in the midline of the anterior chest wall (Figure 1.13). From superior to inferior, the sternum comprises the manubrium sterni, the sternal body and the xiphoid process.

The manubrium connects to the clavicles and part of the second costal cartilages on each side. It sits opposite the 3rd and 4th thoracic vertebrae.

The sternal body connects to the manubrium superiorly with a fibrocartilaginous joint called the sternal angle.

The xiphoid process is a thin hyaline cartilage at the inferior end of the sternal body. It ossifies in adulthood.

Blood supply

The sternum receives blood from the sternal branches of the internal thoracic artery (Tables 1.6 and 1.7).

Nerve supply

The sternum and ribs are innervated by intercostal nerves. These arise from the thoracic nerve roots and run in the neurovascular bundle under each rib. The intercostal nerves also supply the overlying skin, the intercostal muscles and the underlying parietal pleura.