51 Chapter 3

The thoracic cage and the 51 The thoracic cavity, lungs and pleura 52 The heart 56

Clavicle

The thoracic cage and the intercostal space The bony thoracic cage is formed by the 12 at the back, the in front and 12 pairs of in between (Fig. 3.1). The upper seven pairs of ribs articulate anteriorly direct with the sternum through their respective costal cartilages. The costal cartilage of ribs 8, 9 and 10 articulates with that of the above. These ribs with Lower costal the xiphisternum form the lower costal margin. The margin lowermost point of the thoracic cage is the 10th costal cartilage. The space between two adjacent ribs is known as the Fig. 3.1 Surface anatomy of the chest wall. intercostal space. Thus there are 11 intercostal spaces on each side. The junction between the manubrium and the body of the sternum is the sternal angle. The second costal cartilage Clinical box 3.1 articulates at the sternal angle (Figs 3.1, 3.2). This is an important landmark and corresponds to the level of the Rib fractures and ‘stove-in-chest’ lower border of the 4th thoracic . The seventh costal Rib fractures can be fracture of a single rib or can be cartilage anteriorly articulates at the junction between the multiple fractures and are caused by direct blow on the body of the sternum and the xiphisternum. The 8th, 9th and rib or by a crush injury. In a severe crush injury several 10th ribs each articulate with the rib above. The 11th and ribs can fracture in front as well as behind producing a 12th ribs are the fl oating ribs as they have no connection to loose segment of chest wall disconnected from the rest. or cartilage in front. See Clinical box 3.1. This is known as a ‘stove-in-chest’. The loose segment may show paradoxical movements during respiration Surface anatomy i.e. moves inwards during inspiration and blows out The sternal angle is palpable on the surface as a transverse during expiration. Stove-in-chest is a serious condition ridge (Fig. 3.1). This landmark is used to palpate the second needing urgent intubation and positive pressure costal cartilage and the second rib. It is possible to identify ventilation using a respirator as well as a chest drain. the other ribs as well as intercostal spaces by counting down from the second rib. The first rib is not palpable as it is under the clavicle. Ribs 11 and 12 are rudimentary, confined to the back covered by fibres lie in the opposite direction to those of the external. muscles and hence are not palpable. The neurovascular bundle lies between the internal and the innermost intercostal muscles. ✪ If it is necessary to insert a The intercostal space chest drain or a needle into the intercostal space it is always The intercostal space (Fig. 3.3) contains the external placed in the lower part of the space to avoid damage to the intercostal, the internal intercostal and the innermost neurovascular bundle (which lies along the lower border of intercostal muscles arranged in three layers. The the rib along the upper part of the space). The neurovascular neurovascular bundle, consisting of the intercostal nerve bundle consists of, from above downwards, intercostal vein, and vessels, lies in between the internal and the innermost artery and nerve. See Clinical box 3.2. intercostals. The intercostal nerves are the anterior rami of the first 11 The external intercostal muscle fibres are directed thoracic nerves. These supply the intercostal muscles, the downwards and forwards. In the anterior part the muscle skin of the chest wall as well as the parietal pleura. The fibres are replaced by a membrane. The internal intercostal lower intercostal nerves, 7th downwards, supply the

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Suprasternal notch Clinical box 3.2 Thoracocentesis, insertion of a chest drain

Clavicle Insertion of a chest tube into the pleural cavity is required to remove large amounts of serous fl uid, blood, Manubrium sternum pus or air. The site of insertion of the tube is usually at the 5th intercostal space just anterior to the midaxillary Sternal angle line on the affected side. This site will avoid the tube 2nd costal cartilage going through the pectoral muscles which lie more anteriorly and will avoid possible damage of (right Body of sternum side) and (left side) which are overlapped by the pleural cavity more inferiorly (see Clinical box 3.3). Nerve to serratus anterior lies at the level of insertion of Xiphisternum the tube and may be damaged occasionally, causing winging of the scapula (see Clinical box 2.1). A needle thoracocentesis done in a critically ill patient with tension pneumothorax may be life saving. An over 7th costal cartilage the needle catheter is inserted into the pleural cavity on the side of the tension pneumothorax through the second intercostal space in the midclavicular line. 10th costal cartilage Insertion medial to the midclavicular line has a potential danger of damaging the great vessels in the mediastinum. The needle or chest drain is always inserted superior to the rib (lower part of the intercostal space) to avoid damaging the neurovascular bundle. Damage of the Fig. 3.2 Bony thoracic cage. intercostal nerve will cause neuritis and pain (neuralgia) and puncture of the vessels may result in bleeding into Internal intercostal Intercostal the pleural cavity (haemothorax). muscle artery The parietal pleura, the periosteum and other structures in the area of needle insertion and chest drain have rich innervation and hence a good local anaesthesia is required for procedures mentioned above.

lateral to the sternum. In the sixth intercostal space it divides External intercostal muscle into its two terminal branches, the musculophrenic and superior epigastric arteries, the latter entering the anterior abdominal wall by passing through the diaphragm The anterior intercostal arteries are branches of the Intercostal internal thoracic artery or those of its musculophrenic nerve branch. Most of the posterior intercostal arteries are derived Internal from the descending thoracic aorta. ✪ Anastomoses thoracic artery between the anterior and posterior intercostal arteries are Rib important collateral channels for circulation in cases of obstruction to the blood fl ow in the aorta anywhere beyond the origin of the left subclavian artery.

The thoracic cavity, lungs and pleura

Rectus The thoracic cavity contains on either side the right and left abdominus lungs surrounded by the pleural cavities and the Fig. 3.3 Intercostal spaces (left side). mediastinum in between.

The lungs and pleural cavities anterior abdominal wall as well. ✪ Segments of skin See Figures 3.4–3.11. The right lung is subdivided into supplied by the intercostal nerves are common sites of superior, middle and inferior lobes by an oblique fissure and vesicles in Herpes zoster, a viral infection affecting the spinal a horizontal fissure (Figs 3.4 and 3.5). The left lung usually nerve ganglia spreading through the intercostal nerves. has only two lobes, a superior and an inferior with an The internal thoracic artery, a major artery on the anterior oblique fissure in between. Each lung has an apex which aspect of the chest wall, is a branch of the subclavian artery extends about 3cm above the clavicle into the neck, a costal and it descends vertically downwards lying about 1cm surface, a mediastinal surface and a base or diaphragmatic

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Trachea Left common carotid artery Brachiocephalic trunk Left brachiocephalic vein

Upper Upper lobe lobe Anterior border of left lung

Anterior border of right lung

Horizontal fissure Middle lobe Pericardium

Fig. 3.4 The lungs in situ – anterior aspect.

Upper Oesophagus lobe Upper lobe Oblique fissure – left lung Arch of the aorta

Oblique fissure – right lung

Thoracic (descending) aorta Lower lobe Lower lobe Posterior border of right lung Posterior border of left lung

Lower border of right lung Lower border of left lung Right dome of diaphragm Left dome of diaphragm

Fig. 3.5 The lungs in situ – posterior aspect.

surface (Figs 3.6 and 3.7). The anterior border of the lung hangs down in its lower part as the pulmonary ligament. separates the costal and the mediastinal surfaces whereas The right main bronchus gives off the superior lobar the lower border is between the costal and the bronchus outside the lung. All the branches of the left diaphragmatic surface (Fig. 3.6). bronchus are given off inside the lung. The root of the lung The root of the lung connects the lung to the also contains the bronchial arteries supplying the bronchi mediastinum and consists of, anterior to posterior, two and bronchioles, the pulmonary plexus of autonomic nerves pulmonary veins, the pulmonary artery and the bronchus. innervating the lung as well as the lymph nodes draining The pulmonary veins are at a lower level compared with the the lung. The phrenic nerve lies in front of the root of the pulmonary artery (Figs 3.7 and 3.8). The area where these lung and the vagus nerve behind. structures enter the lung is the hilum of the lung. These ✪ The right bronchus is shorter, wider and more vertical structures are enclosed in a sleeve of pleura which loosely than the left. The angle between the two bronchi is about

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Anterior Pulmonary artery Apex border branches

Anterior border Apex Upper lobe

Posterior border

Oblique fissure Superior lobar bronchus Horizontal fissure Oblique fissure Lower lobe Right bronchus

Middle lobe Pulmonary Lower border veins

Oblique fissure

Fig. 3.6 Costal surface of the right lung.

Fig. 3.7 Mediastinal surface of the right lung.

Apex

Groove for arch of aorta Oblique fissure

Left pulmonary artery Left main bronchus Left superior Left inferior pulmonary vein pulmonary vein

Groove for descending aorta Cardiac impression

Oblique fissure

Fig. 3.8 Mediastinal surface of the left lung.

70° in the adult; 25° to the right and 45° to the left from the layer of pleura lines the thoracic cavity and the inner midline. Therefore foreign bodies getting into the trachea visceral or pulmonary layer closely fits on to the surface of tend to go to the right bronchus rather than into the left. At the lung. The two layers become continuous with each birth the bifurcation angle is about 110° with both bronchi other at the root of the lung. The parietal pleura lining the angulating equally from the midline (55° each way). diaphragm is known as the diaphragmatic pleura and that The lung is surrounded by the pleural cavity, the potential lining the mediastinum as the mediastinal pleura. See space between the two layers of pleura. The outer parietal Clinical box 3.3.

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Clavicle

2 2

Sternum

Horizontal fissure Cardiac notch

Lower border Oblique fissure 6 6 of lung

Lower border of pleura

8 8

10 10

Fig. 3.9 Surface relationship of the lungs and pleural cavities. The numbers indicate those of the ribs and costal cartilages.

Clinical box 3.3 Surface anatomy of the lung and pleura Knowledge of the extent of the lung and pleura is pleural cavities are close to each other.) The anterior limit clinically important (Fig. 3.9). Their lower parts overlap of the right pleural cavity descends vertically downwards abdominal organs such as the liver, kidney and spleen. On in the midline from the sternal angle to the level of the the apical pleura lie the subclavian vessels and the sixth costal cartilage. From there the lower border brachial plexus. The stellate ganglion of the sympathetic extends laterally, crossing the eighth rib in the trunk lies behind the apex of the lung and pleura on the midclavicular line, the 10th rib in the midaxillary line neck of the first rib. Pancoast’s tumour affecting the apex and then ascends to the middle of the 12th rib at the of the lung may involve these structures when it spreads back. The posterior border then ascends almost vertically locally. Cannulation of the subclavian vein may upwards in the paravertebral region. A midline inadvertently produce a pneumothorax (air in the sternotomy (splitting of the sternum) is done to open up pleural cavity) resulting in collapse of the lung. the chest cavity for cardiac surgery. During this Procedures such as exposure of the kidney, kidney and procedure the right lung and pleura will be seen liver biopsies may also produce pneumothorax. This is extending up to the midline, and occasionally even due to the fact that the diaphragm is dome shaped and beyond, just behind the sternum. hence the lower parts of the lung and pleura overlap the From the sternal angle the anterior border of the left upper abdominal organs (separated, of course, by the pleural cavity deviates laterally to the lateral border of the diaphragm). sternum. The extent of the lower and the posterior When the lung fields are markedly hyperinfl ated, as in margins are similar to those on the right. emphysema, the liver is pushed down by the diaphragm The surface marking of the lung is the same as that of and may be palpable. the pleura except for the lower margin and the cardiac The apex of the lung and the surrounding pleural cavity notch (Fig. 3.9). The lower margin of the lung is about two extends about 3cm above the medial part of the clavicle. ribs higher than the lower margin of the pleura. Because The apical pleura is covered by a fascia, the suprapleural of the bulge of the heart and pericardium, the anterior membrane (Sibson’s fascia), attached to the inner border border of the left lung deviates laterally from the sternal of the first rib. This fascia prevents the lung and pleura angle to the apex of the heart (usually in the fifth expanding too much into the neck during deep intercostal space a little inside the midclavicular line) inspiration. producing the cardiac notch. The oblique fissure of the From the apex, the anterior border of the pleural cavity lung lies along the sixth rib on both sides and the descends behind the sternoclavicular joint to reach the horizontal fissure of the right lung extends anteriorly midline at the level of the sternal angle. (Here the two from the midaxillary line along the fourth rib.

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Left clavicle

Trachea Right bronchus

Right superior lobe bronchus Left bronchus

Right middle lobe bronchus Left superior Right inferior lobe bronchus lobe bronchus

Left inferior lobe bronchus

Fig. 3.10 Bronchogram – left anterior oblique view.

Clinical box 3.4 Bronchopulmonary segments A bronchopulmonary segment is defined as the area of lung ventilated by a tertiary (branch immediately following the lobar branch) division of the bronchial Respiratory tree. Each segment has its own bronchus and a bronchiole pulmonary artery branch. Pulmonary veins are intersegmental. There are 10 such segments for the right lung and nine for the left. Conditions such as lung Alveolar duct abscess may be localised to these segments and patients can be positioned accordingly to facilitate postural drainage. Secretions collected in anterior segments drain better if the patient lies on the back, and posterior ones Alveolus in the prone position. Lumen of the trachea, main bronchi and the commencement of the segmental bronchi can be visualised during bronchoscopy.

Fig. 3.11 The bronchioles and alveoli.

The alveolar ducts and alveoli The trachea, bronchi and bronchioles Each respiratory bronchiole supplies approximately 200 The trachea, which is slightly to the right of the midline, alveoli via alveolar ducts. There are about 300 million alveoli divides at the carina into right and left main bronchi. in each lung and their walls have type I and type II ✪ The right main bronchus is more vertical than the pneumocytes. Type II pneumocytes are the source of left and, hence, inhaled material is more likely to pass surfactant. The type I pneumocytes and the endothelial cells into it. The right main bronchus divides into three lobar of adjoining capillaries constitute the blood–air barrier, the bronchi (upper, middle and lower), whereas the left only thickness of which is about 0.2–2mm. into two (upper and lower) (Fig. 3.10). Each lobar bronchus divides into segmental and subsegmental bronchi. There are about 25 generations of bronchi and bronchioles The heart between trachea and the alveoli; the first 10 are bronchi and the rest bronchioles (Fig. 3.11). The bronchi have Borders and surfaces of the heart walls consisting of cartilage and smooth muscle, The heart has an anterior or sternocostal surface, formed epithelial lining with cilia and goblet cells, submucosal mostly by the right ventricle, an inferior or diaphragmatic mucous glands and endocrine cells containing surface, formed mostly by the left ventricle, a base or 5-hydroxytryptamine. The bronchioles are tubes less than posterior surface, formed by the left atrium, and an apex, 2mm in diameter and are also known as small airways. formed entirely by the left ventricle. The borders of the heart They have no cartilage or submucosal glands. Their (Fig. 3.12) are the right border, formed by the right atrium, epithelium has a single layer of ciliated cells but only few the inferior border, formed by the right ventricle, the left or goblet cells and Clara cells secreting a surfactant-like obtuse border, formed mostly by the left ventricle with the substance. See Clinical box 3.4. left auricle at its superior end (Fig. 3.13).

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The apex beat is defined as the lower-most and lateral- Blood supply of the heart most cardiac pulsation in the precordium, normally felt The heart muscle is supplied by the right and left coronary inside the midclavicular line in the fifth left intercostal space arteries and is drained by the cardiac veins (Figs 3.14–3.19). (approximately 6cm to the left of the midline) (Fig. 3.13). The coronary arterial supply is of great clinical importance. However it is felt in the anterior axillary line when lying on Its occlusion is the chief cause of death in the western the left side. The right border of the heart extends from the world. third to the sixth right costal cartilage approximately 3cm to The right coronary artery arises from the anterior aortic the right of the midline, the inferior border from the lower sinus. It passes between the pulmonary trunk and the right end of the right border to the apex, and the left border from atrium to lie in the atrioventricular groove (Fig. 3.14). It the apex to the second left intercostal space approximately winds round the inferior border to reach the diaphragmatic 3cm from the midline. See Clinical box 3.5. surface where it anastomoses with the terminal part of the left coronary artery. It gives off an artery to the sinoatrial node, the right (acute) marginal artery and the posterior Blood vessels in the lung Trachea Clavicle Ribs interventricular artery, which is also known as the posterior descending artery (Fig. 3.15).

Clinical box 3.5 Apex beat Apex beat is the lower and lateral-most cardiac pulsation in the precordium, its normal site being just medial to Heart the midclavicular line in the fourth or fifth left shadow intercostal space. It may be normally felt in the anterior axillary line when lying on the left side. There are abnormal forms of apex beats in various clinical conditions. A heaving apex beat which is forceful and sustained may be present in hypertension and aortic stenosis (pressure overload) whereas a thrusting one which is forceful but not sustained is a sign of mitral or aortic regurgitation (volume overload). A tapping apex beat is a sudden but brief pulsation and occurs in mitral stenosis. Diaphragm Diaphragm Apex beat may be missing (i.e. not palpable) in Costo-diaphragmatic recess obesity, pleural effusion, pericardial effusion and emphysema. Fig. 3.12 Posteroanterior radiograph of the chest.

Pulmonary valve A P

Left auricle Aortic valve

Right atrium Mitral valve

Tricuspid valve Left ventricle T Right ventricle M

Fig. 3.13 Surface projections of the heart. A, P, T and M indicate auscultation areas for the aortic, pulmonary, tricuspid and mitral valves.

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Left auricle Ascending aorta Left coronary artery Pulmonary trunk Anterior interventricular artery Right coronary artery Diagonal artery

Right (acute) marginal artery Obtuse (left) marginal artery

Left ventricle Right ventricle

Apex

Fig. 3.14 Coronary arteries – anterior aspect of the heart.

Right atrium Left atrium

Inferior vena cava Coronary sinus

Circumflex artery Left ventricle Middle cardiac vein Obtuse (left) marginal artery

Right ventricle Posterior interventricular artery

Right (acute) marginal artery

Anterior interventricular artery

Fig. 3.15 Coronary arteries – posteroinferior aspect of the heart.

The left coronary artery arises from the left posterior population in whom the left coronary is larger and longer aortic sinus. It passes behind the pulmonary trunk and the than usual – ‘left dominance’ – the posterior descending left auricle to reach the atrioventricular groove where it artery arises from it instead of from the right coronary. divides into the circumfl ex and the anterior interventricular Another 10% have ‘co-dominant’ coronary circulation (anterior descending) arteries, both of equal size (Figs where both left and right coronaries contribute equally to 3.14, 3.15). The circumflex artery winds round the left the posterior interventricular artery. In a third of the margin where it gives off the left (obtuse) marginal artery population the left main stem divides into three branches and reaches the diaphragmatic surface to anastomose instead of two, the third being a branch lying between the with the right coronary artery. The anterior descending circumflex and the anterior descending on the lateral artery (LAD), also known as the ‘widow maker’ because aspect of the left ventricle. many men die of blockage of this artery, descends in the The blood supply of the conducting system is of clinical interventricular septum and gives off ventricular importance. In about 60% of the population the sinoatrial branches, septal branches as well as the diagonal artery. It node is supplied by the right coronary and in the rest by the then winds round the apex reaching the diaphragmatic circumfl ex branch of the left coronary. However occasionally surface to anastomose with the posterior descending (3%) it can have a dual supply. The atrioventricular node is artery. The main stem of the left coronary artery varies in supplied by the right coronary in 90% and the circumfl ex length between 4mm and 10mm. In 10% of the in 10%.

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Left coronary artery: RAO view

Right coronary artery Left anterior descending LAD RAO view

Main RCA 1st diagonal L1 main stem

Septals

Right ventricular branch Circumflex

Apex Circumflex Posterior descending artery

Fig. 3.16 Right coronary arteriogram – right anterior oblique view. Fig. 3.18 Left coronary arteriogram – right anterior oblique view.

Left coronary artery: lateral view

T. spine

Upper ventricular Sternum

Diagonal

Right ventricular branch

Posterior descending artery Circumflex

Right coronary artery: LAO view Septals Obtuse marginal LAD

Fig. 3.17 Right coronary arteriogram – left anterior oblique view. Fig. 3.19 Left coronary arteriogram – lateral view.

Cardiac veins accompany the arteries. Most of them are and function to the pleural cavity. The pericardium provides tributaries of the coronary sinus, a sizable vein lying in the a friction-free surface for the heart to accommodate its posterior part of the atrioventricular groove and opening sliding movements. into the right atrium. The great cardiac vein accompanies Components of the pericardium are the fibrous the anterior interventricular artery; the middle cardiac vein pericardium and the serous pericardium, the former being a accompanies the posterior interventricular artery and the collagenous outer layer fused with the central tendon of the small cardiac vein accompanies the marginal artery. Anterior diaphragm. The serous pericardium consists of a parietal cardiac veins seen on the anterior wall of the right ventricle layer which lines the inner surface of the fibrous drain directly into the right atrium. Additionally there are pericardium and a visceral layer which lines the outer very small veins on the various walls – venae cordis surface of the heart and the commencement of the great minimae, draining directly into the cardiac cavity. See vessels. The pericardial cavity is the space between the Clinical box 3.6. parietal and the visceral layers. Two regions of the pericardial cavity have special names. The pericardium The transverse sinus of the pericardial cavity lies between The heart lies within the pericardial cavity, in the middle the ascending aorta and the pulmonary trunk in front and mediastinum. The pericardial cavity is similar in structure the venae cavae and the atria behind. The pericardial space

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Clinical box 3.6 Coronary artery disease Occlusion of a coronary artery or its branch causes Coronary arteries and their branches can be visualised myocardial infarction which is cell death of the cardiac by selectively catheterising each coronary artery and musculature due to inadequate blood supply. A partial injecting a radio-opaque dye (usually iodine-containing). occlusion may manifest as angina which typically is felt as Several procedures are now available to treat coronary a deep pain in the sternal area radiating to the left arm artery disease. In an angioplasty a catheter with a small and left side of the neck. infl atable balloon attached to its tip is passed into the The changes caused by occlusion are based on the coronary artery (via the femoral, external and common distribution of the coronary artery branches. Right iliac and aorta). The balloon is infl ated to widen the artery coronary artery occlusion leads to inferior myocardial by fl attening the atheromatous plaque. In the coronary infarction, often associated with dysrhythmia artery bypass graft operation a small segment of the great (abnormal heart beats) due to ischaemia of SA node saphenous vein is connected to the ascending aorta or to and/or AV node, parts of the conducting system. the coronary artery proximal to the obstruction and the Occlusion of the left coronary artery or its branches distal end of the segment is then attached to the coronary leads to anterior and/or lateral myocardial infarction, artery distal to the narrowing bypassing the obstruction. often with substantial ventricular damage and very The radial artery and the internal thoracic artery are also poor prognosis. commonly used for bypass surgery.

Pulmonary vein

Left atrium Left ventricle Oblique sinus

Parietal layer of pericardium Inferior vena cava lining the fibrous pericardium

Fig. 3.20 Pericardial cavity opened up and the heart lifted up to show the oblique sinus.

behind the left atrium is the oblique sinus (Fig. 3.20). The oblique sinus separates the left atrium from the oesophagus. Clinical box 3.7 Anteriorly the pericardium is related to the sternum, third Pericardiocentesis to sixth costal cartilages, lungs and the pleura. Posterior relations are oesophagus, descending aorta and T5–T8 Diseases of the pericardium can cause accumulation of vertebrae. Laterally on either side lie the root of the lung, fl uid or blood in the pericardial cavity. Blood can also mediastinal pleura and the phrenic nerve. Innervation of the accumulate in the pericardial cavity as a result of fibrous and the parietal layer of serous pericardium is by the trauma. To remove fl uid or blood from the pericardial phrenic nerves. Pericardial pain originates in the parietal cavity a needle is inserted into the angle between the layer and is transmitted by the phrenic nerves. The and the left seventh costal cartilage and pericardial cavity is closest to the surface at the level of the is directed upwards at an angle of 45° towards the left xiphoid process of sternum and the sixth costal cartilages. shoulder. The needle passes through the central tendon See Clinical box 3.7. of the diaphragm before entering the pericardial cavity.

Interior of the chambers of the heart which bring systemic venous blood into the smooth part of The right atrium the atrium. The coronary sinus opens anterior to the The right atrium (Fig. 3.21) has a smooth and a rough part opening of the inferior vena cava. Developmentally the which are separated by a vertical ridge, the crista terminalis, smooth part of the atrium is derived from the sinus venosus extending between the superior and inferior venae cavae of the primitive cardiac tube and the rough part which has

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muscular ridges known as musculae pectinatae from the (Fig. 3.22). The chordae tendineae connect the papillary primitive atrium. The fossa ovalis (Fig. 3.21), an oval muscles to the tricuspid valve cusps. ✪ These prevent the depression on the interatrial wall, is the remnant of the valve cusps being everted into the atrium during ventricular foramen ovale in the fetus. Before birth the foramen ovale systole. Failure of this mechanism due to breakage of the allowed blood to fl ow from the right atrium to the left papillary muscle or chordae tendineae causes tricuspid atrium bypassing the lungs. At birth when the lungs begin incompetence and regurgitation of blood back into the to function the foramen ovale closes to produce the fossa atrium during ventricular systole. When this happens blood ovalis. from the atrium can pool back into the liver and the neck veins causing enlarged neck veins and palpable liver as the superior and inferior venae cavae do not have valves. The right ventricle The septomarginal trabecula (moderator band) is a The right ventricular wall is thicker than that of the muscular ridge extending from the interventricular septum atrium. The tricuspid orifice is guarded by the tricuspid to the base of the anterior papillary muscle of the heart. The valve which has an anterior, posterior and a septal cusp. The moderator band is a part of the conducting system of the interior of the ventricle has muscular ridges known as heart which regulates the cardiac cycle. trabeculae carneae as well as the anterior, posterior and The infundibulum leads on to the orifice of the septal (small) papillary muscles and the chordae tendineae pulmonary trunk. The pulmonary orifice has the pulmonary valve with three semilunar cusps. Each cusp has a thickening in the centre of its free edge.

The left atrium The left atrium which develops by a combination of absorption of the pulmonary veins as well as from the Right auricle primitive atrium has the openings of the four pulmonary veins. The mitral orifice separates the left atrium from the Superior vena cava left ventricle.

The left ventricle The walls of the left ventricle are about three times thicker Musculi pectinati than those of the right ventricle because of the increased resistance of the systemic circulation compared with that of Crista terminalis the pulmonary circulation. The mitral orifice is guarded by the mitral valve with an anterior and a posterior cusp. The Coronary sinus large anterior cusp lies between the aortic and mitral Inferior vena cava orifices. The trabeculae carneae, papillary muscles and chordae tendineae are similar to those in the right ventricle. The aortic orifice has the aortic valve (Fig. 3.23) with the three semilunar aortic cusps, one anterior and two posterior in the anatomical position of the heart. These are thicker than those of the pulmonary valves to cope with the Fossa ovalis increased pressure. Alongside each cusp there is a dilation, Fig. 3.21 Interior of the right atrium. the aortic sinus. The coronary arteries originate from the

Cusps of pulmonary valve

Infundibulum Trabeculae carneae Interventricular septum

Anterior cusp of tricuspid valve Septal cusp of tricuspid valve

Chordae tendineae Posterior cusp of tricuspid valve Anterior papillary muscle Interventricular septum

Posterior papillary muscle

Fig. 3.22 Interior of the right ventricle.

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Aorta

Anterior (right coronary) sinus Aortic valve cusps Aortic vestibule Anterior cusp of mitral valve Chordae tendineae Posterior cusp of mitral valve

Papillary muscles

Fig. 3.23 Interior of the left ventricle.

Clinical box 3.8 Clinical box 3.9 Valves, heart sounds and murmurs Areas of auscultation The valves between the atria and the ventricles, i.e. the The two heart sounds and the abnormal murmurs are tricuspid and the mitral valves, prevent regurgitation of caused by turbulence and vibrations inside the blood from the ventricles back into the atria during ventricles, the aorta or the pulmonary trunk. This is best ventricular contraction (systole). Similarly the heard where the particular chamber or vessel is closer to pulmonary and aortic valves prevent regurgitation the surface. Thus the mitral valve closure produces during diastole (relaxation of ventricle) from these vibrations in the left ventricle and the sound is best vessels back into the ventricles. Closure of the tricuspid heard where the left ventricle is closer to the surface, i.e. and mitral valves occurs at the beginning of systole and where the apex beat is felt. Mitral valve therefore is causes the first heart sound and closure of the aortic and auscultated at the apex, tricuspid at the lower end of pulmonary valves, which happens at the beginning of sternum pulmonary valve at the second intercostal diastole, the second sound. Thus the interval between space on the left side just outside the lateral border of the first and the second heart sounds is the period of sternum, and the aortic valve in the second intercostal ventricular systole and that between the second and the space close to the lateral border of the sternum on the next first sound is the diastole. A hissing sound heard right side (Fig. 3.13). during systole is a systolic murmur and that during diastole is a diastolic murmur. Murmurs are caused by blood fl ow through narrow orifice or leaking valves. SA node AV node Left bundle branch Pulmonary or aortic valve stenosis (narrowing) cause systolic murmur. It can also be heard in mitral or tricuspid incompetence (regurgitation). A diastolic murmur, on the other hand, is a characteristic of mitral or tricuspid stenosis. It is also a sign of aortic or pulmonary valve incompetence.

sinuses, the right from the anterior (also known as the right coronary sinus) and the left from the left posterior aortic sinus (also known as the left coronary sinus). The interventricular septum which has the muscular and the membranous parts bulges into the right ventricle and separates the left ventricle from the right. See Clinical boxes 3.8 and 3.9.

The conducting system of the heart Specialised cardiac muscle cells initiate and regulate the Atrioventricular Right bundle heart-beat. The sinoatrial node (SA node) or ‘pacemaker of bundle branch the heart’ initiating the heart-beat is situated in the right atrium at the upper end of the crista terminalis (Fig. 3.24). Fig. 3.24 The conducting system of the heart. From there the cardiac impulse spreads through the atrial musculature to reach the AV node (atrioventricular node) through the fibrous ring at the atrioventricular junction to which is situated in the interatrial septum near the opening reach the membranous part of the interventricular septum of the coronary sinus. After a brief pause there the impulse where it divides into a right and left bundle branch. The passes into the atrioventricular bundle of His (AV bundle). atrioventricular bundle is the only pathway through which The AV bundle which starts from the AV node passes impulses can reach the ventricles from the atrium. The left

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Right vagus

Right brachiocephalic vein

Trachea Left brachiocephalic vein Superior vena cava Azygos vein Right phrenic nerve

Right bronchus

Right sympathetic Branches of right trunk pulmonary artery

Splanchnic nerves

Oesophagus Right pulmonary veins

Pericardium

(A) Sympathetic trunk Oesophagus

Trachea Azygos vein

Right brachiocephalic vein

Right vagus Arch of aorta

Superior vena cava Superior lobe bronchus

Pulmonary artery

Right bronchus Pulmonary veins

Greater splanchnic nerve Right phrenic nerve

(B)

Fig. 3.25 a & b Right side of the mediastinum after removal of the right lung and pleura. Viewed from the right side.

and right bundles descend towards the apex and break up superior mediastinum lies above the horizontal plane into Purkinje fibres which activate the musculature of the joining the sternal angle to the lower border of T4 vertebra. ventricle in such a way that the papillary muscles contract The middle mediastinum contains the heart and first followed by the simultaneous contraction of both the pericardium; the anterior mediastinum is in front of this ventricles from apex towards the base. and the posterior mediastinum behind.

The mediastinum The brachiocephalic vein and the superior vena cava The mediastinum is the region between the two pleural The brachiocephalic vein, one on each side, is formed by the cavities. It contains the heart, great vessels, trachea, union of the subclavian and the internal jugular veins. The right oesophagus and many other structures. The mediastinum is and left brachiocephalic veins join together to form the superior divided into four parts for descriptive purposes. The vena cava which drains into the right atrium (Fig. 3.25).

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Left subclavian artery

Left superior. intercostal vein Left vagus Left phrenic nerve Arch of the aorta

Left pulmonary artery

Descending thoracic aorta

Pericardium Left sympathetic trunk

Greater splanchnic nerve

(A) Left subclavian artery Oesophagus

Left common carotid artery Left vagus nerve

Arch of aorta

Left recurrent laryngeal nerve

Left phrenic nerve

Descending thoracic aorta

(B)

Fig. 3.26 A & B Left side of the mediastinum.

The azygos vein which receives segmental veins from the right side of the heart and pericardium (where it lies in front thoracic and posterior abdominal walls (intercostal and of the root of the lung) and the inferior vena cava. In other lumbar veins) joins the superior vena cava. words it lies on the big veins and the right atrium. The left phrenic nerve crosses the arch of the aorta (Figs The phrenic nerves 3.26, 3.27). It descends in front of the root of the lung then The right and left phrenic nerves are formed in the cervical lies on the pericardium as it descends to reach the plexus (C3, 4, 5). Besides supplying the diaphragm they give diaphragm sensory innervation to pleura, pericardium and peritoneum (all starting with ‘p’!). The thoracic part of the right phrenic The right and left vagus nerves nerve (Fig. 3.25) reaches the diaphragm lying on the surface The right vagus nerve lies on the trachea (Fig. 3.25) and of the right brachiocephalic vein, the superior vena cava, the crosses behind the root of the lung and breaks up into

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Interior thyroid veins

Trachea Right brachiocephalic vein

Brachiocephalic trunk (artery) Left subclavian artery Left brachiocephalic vein Left common carotid artery Left vagus

Arch of aorta Left recurrent laryngeal nerve

Superior vena cava

Ascending aorta Pulmonary trunk Right phrenic nerve

Left phrenic nerve Right lung

Heart and pericardium

Fig. 3.27 Structures in the superior mediastinum seen after removal of the thoracic cage and the parietal pleura. The lungs have been retracted to expose the structures.

branches on the oesophagus forming the oesophageal plexus. It leaves the thorax by passing along with the Clinical box 3.10 oesophagus through the diaphragm as the posterior gastric Arch of the aorta nerve. The left vagus, like the left phrenic nerve, crosses the arch The arch of the aorta hooks over the left bronchus and of the aorta (Figs 3.26, 3.27). It crosses behind the root of the lies on the left side of the trachea and oesophagus with left lung (the phrenic nerve descends in front). The left the left recurrent laryngeal nerve lying between the two. vagus gives off an important branch, the left recurrent An aneurysm of the arch of the aorta can occlude the laryngeal nerve, as it crosses the arch of the aorta. The left left bronchus and collapse the left lung. It can produce a recurrent laryngeal nerve winds round the ligamentum change in voice due to compression of the left recurrent arteriosum, a fibrous connection between the left laryngeal nerve. Pathology of the aorta, trachea, pulmonary artery and the arch of the aorta. The ligamentum bronchus and the oesophagus tend to involve one arteriosum is the remnant of the ductus arteriosum which another due to their close relationship. Pulsation of the shunts blood from the pulmonary trunk to the aorta in the arch of the aorta is visible during bronchoscopy and fetus. The recurrent laryngeal nerve ascends to the neck oesophagoscopy. lying in the groove between the trachea and the oesophagus and supplies the muscles and mucous membrane of the larynx. Carcinoma of the oesophagus, mediastinal lymph node confined to the superior mediastinum. It has three branches: enlargement and aortic arch aneurysm may compress the the brachiocephalic trunk which divides into the right left recurrent laryngeal nerve to cause change in voice. common carotid and the right subclavian arteries, the left Below the root of the lung the left vagus, like the right, common carotid artery and the left subclavian artery (Fig. breaks up into branches contributing to the oesophageal 3.28). The left vagus and the left phrenic nerves cross the arch plexus and leaves the thorax by passing along with the of the aorta. The small vein lying across the arch of the aorta oesophagus through the diaphragm as the anterior gastric is the left superior intercostal vein. This drains the second nerve. and third left intercostal spaces and in turn drains into the left brachiocephalic vein (Fig. 3.26). See Clinical box 3.10. Arch of the aorta The ascending aorta commencing from the left ventricle The trachea continues upwards and to the left over the root of the left The trachea (Figs 3.29, 3.30) extends from the lower border lung as the arch of the aorta (Figs 3.26–3.28). It then of the cricoid cartilage in the neck to the tracheal bifurcation descends down to become the descending thoracic aorta. at the level of the lower border of the T4 vertebra. In the The arch of the aorta commences at the level of the sternal living, in the erect posture, the tracheal bifurcation is at a angle and ends at the lower border of T4. It is entirely lower level. The trachea is about 15cm long, the first 5cm

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Left common carotid artery

Right common carotid artery

Right vertebral artery

Left vertebral artery Right common carotid artery

Left subclavian artery Right subclavian artery Left common carotid artery

Right internal thoracic artery Brachiocephalic trunk

Arch of aorta

Fig. 3.28 Arch aortogram.

Right vagus

Right recurrent Trachea laryngeal nerve

Right vagus Left vagus

Left recurrent laryngeal nerve Right phrenic nerve

Arch of aorta (cut) Left phrenic nerve

Superior vena cava Ligamentum arteriosum

Pulmonary trunk Ascending aorta

Tracheal bifurcation

Fig. 3.29 Superior mediastinum – deeper aspect. Part of the arch of the aorta and its branches, the superior vena cava and the brachiocephalic veins have been removed.

being in the neck. The cervical part of the trachea lies in the bridged by the trachealis muscle which allows the trachea midline and is easily palpable. to constrict and dilate. It is elastic enabling it to stretch The diameter of the lumen of the trachea is correlated to during swallowing and its diameter changes during the size of the subject and has approximately the same coughing and sneezing. diameter as his/her index finger. It is made up of 15–20 The thoracic part of the trachea is in the superior ‘C’-shaped cartilaginous rings which prevent it from mediastinum. Anteriorly it is related to the left collapsing. The gap in the cartilage is at the back and is brachiocephalic vein, the commencement of the

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