Interior of the chambers of the heart
The right atrium
The right atrium has a smooth and a rough part which are separated by a vertical ridge, the crista terminalis, extending between the superior and inferior venae cavae which bring systemic venous blood into the smooth part of the atrium.
The coronary sinus opens anterior to the opening of the inferior vena cava.
Developmentally the smooth part of the atrium is derived from the sinus venosus of the primitive cardiac tube and the rough part which has muscular ridges known as musculae pectinatae from the primitive atrium.
The fossa ovalis, an oval depression on the interatrial wall, is the remnant of the foramen ovale in the fetus.
The right ventricle
The right ventricular wall is thicker than that of the atrium.
The tricuspid orifice is guarded by the tricuspid valve which has an anterior, posterior and a septal cusp.
The interior of the ventricle has muscular ridges known as trabeculae carneae as well as the anterior, posterior and septal (small) papillary muscles and the chordae tendineae.
The chordae tendineae connect the papillary muscles to the tricuspid valve cusps.
These prevent the valve cusps being everted into the atrium during ventricular systole.
The septomarginal trabecula (moderator band) is a muscular ridge extending from the interventricular septum to the base of the anterior papillary muscle of the heart.
The moderator band is a part of the conducting system of the heart which regulates the cardiac cycle.
The infundibulum leads on to the orifice of the 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 primitive atrium has the openings of the four pulmonary veins.
The mitral orifice separates the left atrium from the left ventricle.
The left ventricle
The walls of the left ventricle are about three times thicker than those of the right ventricle because of the increased resistance of the systemic circulation compared to that of the pulmonary circulation.
The mitral orifice is guarded by the mitral valve with an anterior and a posterior cusp.
The large anterior cusp lies between the aortic and mitral orifices.
The trabeculae carneae, papillary muscles and chordae tendineae are similar to those in the right ventricle.
The aortic orifice has the aortic valve 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 increased pressure.
Alongside each cusp there is a dilation, the aortic sinus.
The coronary arteries originate from the sinuses, the right from the anterior and the left from the left posterior aortic 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.
The conducting system of the heart
The sinoatrial node (SA node) or ‘pacemaker of the heart’ is situated in the right atrium at the upper end of the crista terminalis.
From there the cardiac impulse spreads through the atrial musculature to reach the AV node (atrioventricular node) which is situated in the interatrial septum near the opening of the coronary sinus.
From there the atrioventricular bundle of His (AV bundle) passes through the fibrous ring at the atrioventricular junction to reach the membranous part of the interventricular septum where it divides into a right and left bundle branch.
The atrioventricular bundle is the only pathway through which impulses can reach the ventricles from the atrium.
The left and right bundles descend towards the apex and break up into Purkinje fibres which activate the musculature of the ventricle in such a way that the papillary muscles contract first followed by the simultaneous contraction of both the ventricles from apex towards the base.
The mediastinum
The mediastinum is the region between the two pleural cavities.
It contains the heart, great vessels, trachea, oesophagus and many other structures.
The mediastinum is divided into four parts for descriptive purposes.
The superior mediastinum lies above an imaginary line joining the sternal angle to the lower border of T4.
The middle mediastinum contains the heart and pericardium; the anterior mediastinum is in front of this and the posterior mediastinum behind.
The brachioc ephalic vein and the superior vena cava
The brachiocephalic vein, one on each side, is formed by the union of the subclavian and the internal jugular veins.
The right and left brachiocephalic veins join together to form the superior vena cava which drains into the right atrium.
The azygos vein which receives segmental veins from the thoracic and posterior abdominal walls (intercostal and lumbar veins) joins the superior vena cava.
The right phrenic nerve
The right and left phrenic ner ves are formed in the cervical plexus (C3,4,5).
Besides supplying the diaphragm they give sensory innervation to pleura, pericardium and peritoneum.
The thoracic part of the right phrenic nerve reaches the diaphragm lying on the surface of the right brachiocephalic vein, the superior vena cava, the right side of the heart and pericardium (where it lies in front of the root of the lung) and the inferior vena cava.
The right vagus nerve
The right vagus nerve lies on the trachea and crosses behind the root of the lung and breaks up into branches on the oesophagus.
It leaves the thorax by passing along with the oesophagus through the diaphragm as the posterior gastric nerve.
The left vagus and the left phrenicnerves
Both nerves cross the arch of the aorta.
The phrenic nerve descends in front of the root of the lung whereas the vagus crosses behind it.
The left vagus gives off an important branch, the left recurrent laryngeal nerve, as it crosses the arch of the aorta.
The left recurrent laryngeal nerve winds round the ligamentum arteriosum, a fibrous connection between the left pulmonary artery and the arch of the aorta.
The ligamentum arteriosum is the remnant of the ductus arteriosum which shunts blood from the pulmonary trunk to the aorta in the fetus.
The recurrent laryngeal nerve ascends to the neck 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 enlargement and aortic arch aneurysm may compress the left recurrent laryngeal nerve to cause change in voice.
Arch of the aorta
The ascending aorta commencing from the left ventricle continues upwards and to the left over the root of the left lung as the arch of the aorta.
It then descends down to become the descending thoracic aorta. The arch of the aorta is entirely confined to the superior mediastinum.
It has three branches: the brachiocephalic trunk which divides into the right common carotid and the right subclavian arteries, the left common carotid artery and the left subclavian artery.
The left vagus and the left phrenic nerves cross the arch of the aorta.
The small vein lying across the arch of the aorta is the left superior intercostal vein.
This drains the second and third left intercostal spaces and in turn drains into the left brachiocephalic vein.
The trachea
The trachea extends from the lower border of the cricoid cartilage in the neck to the tracheal bifurcation at the level of the lower border of the T4 vertebra.
In the living, in the erect posture, the tracheal bifurcation is at a lower level.
The trachea is made up of a series of C-shaped cartilages closed posteriorly by the trachealis muscle.
It is elastic enabling it to stretch during swallowing and its diameter changes during coughing and sneezing.
The cervical part of the trachea lies in the midline and is easily palpable The angle of bifurcation of the trachea in the adult is such that the right main bronchus is more vertical than the left.
The cartilage at the tracheal bifurcation is the keel-shaped carina which projects into the lumen as a vertical ridge.
Flattening of this ridge is a sign of alteration of the bifurcation angle, often due to enlargement of the tracheobronchial group of lymph nodes.
The angle of bifurcation of the trachea is normally wider in children, with no difference in the angulation of the right and left main bronchi.
The descending (thoracic) aorta
The descending aorta commences where the arch of the aorta ends at the lower border of T4 vertebra.
It leaves the posterior mediastinum in the midline at the level of T12 vertebra by passing between the crura of the diaphragm.
The descending aorta gives off nine pairs of posterior intercostal arteries, a pair of subcostal arteries, two bronchial arteries for the left lung and small branches to the oesophagus.
Radicular arteries arise from the posterior intercostal arteries and supply the spinal cord.
One such artery (usually from the tenth or eleventh intercostal space) is large and is known as the great radicular artery or artery of Adamkiewicz.
Blood flow through radicular arteries may be interfered with during aortic surgery producing ischaemia of the spinal cord resulting in paraplegia.
The oesophagus
The oesophagus starts as a continu ation of the pharynx at the level of C6 vertebra and ends by entering the stomach at the cardiac orifice.
The thoracic part of the oesophagus lies in the superior and posterior mediastinum and enters the abdomen by piercing the diaphragm at the level of T10 vertebra.
In the superior mediastinum it lies behind the trachea with the arch of the aorta lying on its left side.
The left recurrent laryngeal nerve lies in the groove between the trachea and the oesophagus.
The left main bronchus crosses in front of the oesophagus and the part below that is related to the left atrium.
The arch of the aorta, the left main bronchus and the left atrium produce indentations on the oesophagus which can be seen clearly on radiograph taken after barium swallow.
The close relationship of the oesophagus and the left atrium is made use of in determining left atrial enlargement in mitral stenosis.
Barium swallow may show displacement of the oesophagus by the enlarged atrium.
The lumen of the oesophagus is narrower at its commencement, where the left bronchus crosses it and where it passes through the diaphragm to enter the stomach.
These are sites where foreign bodies swallowed are usually impacted and where strictures develop after swallowing caustic fluids. They are also common sites for carcinoma of the oesophagus.
Thoracic duct
The thoracic duct starts as the continuation of the cysterna chyli in the abdomen, passes through the thorax and enters the neck lying on the left border of the oesophagus.
In the neck it arches to the left, lying in the plane between the carotid sheath and vertebral arteries, to enter the junction between the subclavian and internal jugular veins.
It carries lymph from the whole body except that from the right side of thorax, right upper limb, right side of head and neck and the lower lobe of right lung.
The azygos vein
The azygos vein enters the thorax through the aortic opening of the diaphragm and passes upwards lying on the vertebral bodies and arches over the root of the right lung to drain into the superior vena cava.
The azygos vein receives the lower eight posterior intercostal veins of the right side and the right superior intercostal vein.
The hemiazygos veins which receive the intercostal veins of the left side drain into the azygos vein.
The sympathetic trunk
The sympathetic trunks lie on each side of the vertebral column, extending from the base of the skull to the coccyx where the two chains fuse together. Each trunk contains a number of sympathetic ganglia, the thoracic region having about 11 ganglia which lie on the neck of the ribs.
The ganglia are closely related to the intercostal nerves from which they receive preganglionic fibres as white rami communicantes.
The post-ganglionic fibres from the ganglia go back to the intercostal nerves as grey rami communicantes.
The thoracic ganglia give off the greater, lesser and least splanchnic nerves to supply the abdominal viscera.
The splanchnic nerves are pre-ganglionic fibres which will synapse in collateral ganglia displaced from the sympathetic trunk (e.g. coeliac ganglion) in the abdomen.
Diaphragm
The diaphragm separates the thoracic and abdominal cavities.
It transmits the inferior vena cava, the oesophagus, the sympathetic trunk and the splanchnic nerves.
The aorta passes behind the diaphragm between its two crura.
The peripheral part of the diaphragm is muscular whereas its central part, the central tendon, is fibrous. Its upper surface fuses with the fibrous pericardium.
The muscular part is attached to the upper lumbar vertebrae and the intervertebral discs through the right and left crura.
Fibres of the diaphragm also take origin from the medial and lateral arcuate ligaments which are thickenings of the fascia overlying the psoas major and the quadratus lumborum muscles on the posterior abdominal wall.
Besides these vertebral attachments the diaphragm is attached to the inner aspects of the lower six ribs, and costal cartilages as well as to the xiphoid process of the sternum.
The aortic opening lies at the level of T12 and transmits the abdominal aorta, thoracic duct and the azygos vein.
The oesophagus passes through the left crus with the fibres of the right crus looping around it at the level of T10. The oesophageal orifice also transmits the vagus nerves and the left gastric vessels.
The inferior vena caval opening which lies more anteriorly in the central tendon of the diaphragm at the level of T8 also transmits the right phrenic nerve.
The sympathetic trunk enters the abdomen by passing under the medial arcuate ligament and the splanchnic nerves by piercing the crura.
The nerve supply of the diaphragm, both sensory and motor, is by the phrenic nerve and hence pain due to diaphragmatic irritation is felt in the shoulder region as a referred pain.
There is additional supply of sensory nerves to the peripheral aspect by the intercostal nerves.
On contraction the diaphragm descends down to increase the vertical diameter of the thoracic cavity. It thus acts as the major muscle of inspiration.
The abdominal pressure is increased by contraction of the diaphragm and hence it contributes importantly to functions such as defecation, micturition and parturition.
Injury to the phrenic ner ve will paralyse the corresponding half of the diaphragm resulting in paradoxical movement during respiration.
Instead of descending during inspiration the paralysed side gets pushed upwards by the abdominal viscera.
This can be detected radiographically.
