The Mediastinum & Heart

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1 2018 The Mediastinum & Heart Sameh S. Akkila

2 THE MEDIASTINUM The mediastinum is a septum that lies between the two lungs. It extends from the vertebral column posteriorly to the sternum anteriorly and from the superior thoracic aperture above to the diaphragm below. It is enclosed by the mediastinal pleura on the sides. For descriptive purposes, the mediastinum is divided into several smaller regions. A plane passing through the sternal angle and the disc between the T4 and T5 vertebrae divides the mediastinum into superior and inferior parts. The inferior mediastinum is subdivided by the heart into anterior, middle and posterior parts. The heart represents the middle part of the inferior mediastinum. The superior mediastinum may also be subdivided into three parts: a retrosternal part; immediately posterior to the manubrium sterni, a prevertebral part; just anterior to the vertebral column and an intermediate part in the middle. Contents of one part of the mediastinum may extend into another part or travel to and from the neck or abdominal cavity. The contents of each part of the mediastinum are; Figure 1. The parts of the mediastinum (R: retrosternal, I: intermediate, P: prevertebral). Contents of the superior mediastinum; from anterior to posterior: The retrosternal part contains the thymus gland, brachiocephalic veins and superior vena cava. The intermediate part contains the phrenic nerves, the aortic arch and its branches and the vagus nerves. The prevertebral part contains the trachea, left recurrent laryngeal nerve, esophagus, thoracic duct, upper part of the azygos vein and thoracic part of the sympathetic trunk. 1

3 Contents of the inferior mediastinum The Anterior mediastinum contains the sternopericardial ligaments, fat and the lower part of the thymus gland. The middle mediastinum contians the heart, pericardium and great vessels attached to the heart. The posterior mediastinum conttains the esophagus, descending aorta, thoracic duct, hemiazygos veins and lower parts of the azygos vein and thoracic sympathetic trunk. Figure 2. Horizontal section through the superior mediastinum at the level of T2 vertebra, superior view. The superior mediastinum extends from the manubrium sterni anteriorly to the bodies of the upper four thoracic vertebrae posteriorly. Its upper limit is an oblique plane that passes from the suprasternal notch anteriorly to the upper border of T1 vertebra posteriorly. Its lower limit is the plane of the sternal angle. The arrangement of the contents of the superior mediastinum is best studied in a horizontal section passing through the body of T2 or T3 vertebra as shown in. Contents of the prevertebral part of the superior mediastinum will be studied in continuity with structures of the posterior mediastinum. The anterior and middle parts of the inferior mediastinum are discussed separately with the heart and pericardium. The Thymus Gland The thymus gland is a primary (central) lymphoid organ that consists of two elongated, encapsulated but asymmetrical lobes. The gland extends from the lower part of the neck to the upper part of the anterior mediastinum. In the neck, it may reach as high as the thyroid gland. In the superior mediastinum, the thymus lies posterior to the manubrium sterni, anterior to the brachiocephalic veins and between the diverging anterior borders of the lungs and pleurae. In the anterior mediastinum, it lies between the sternal body anteriorly and the pericardium posteriorly. Because the thymus is involved in early development of the immune system and is highly hormone-sensitive, it is a large structure in the child, begins to atrophy after puberty, and shows considerable size variation in the 2

4 adult. In the elderly, it is barely identifiable as an organ, consisting mostly of fatty tissue that is sometimes arranged as two lobulated fatty structures. However, it remains functional throughout life. The function of the gland as a primary lymphoid organ is to educate the newly formed lymphocytes in differentiating body molecules from foreign antigens. These lymphocytes are formed in in the bone marrow and reach the thymus via blood vessels. The mature T-lymphocytes leave the gland via efferent lymph vessels. Therefore, the thymus has efferent but no afferent lymph vessels. The thymus receives its blood supply by small branches from the inferior thyroid and internal thoracic arteries. Venous drainage is usually into the left brachiocephalic vein and possibly into the internal thoracic veins. The Brachiocephalic Veins and the Superior Vena Cava The right and left brachiocephalic veins are located immediately posterior to the thymus. Each brachiocephalic vein begins posterior to the corresponding sternoclavicular joint by the union of the internal jugular and subclavian veins. The right brachiocephalic vein descends vertically to the level of the 1 st intercostal space where it joins the left brachiocephalic vein to form the superior vena cava. The left brachiocephalic vein is twice as long. It descends obliquely behind the upper 1 / 2 of the manubrium sterni to reach the right 1 st intercostal space and unite with the right brachiocephalic vein. The superior vena cava descends vertically from the point of union of the brachiocephalic veins to the level of the right 3 rd intercostal space where it enters the right atrium of the heart. Before entering the heart, the superior vena cava receives the azygos arch at the level of the sternal angle. Figure 3. Anterior view of the major vessels and nerves of the superior mediastinum. Tributaries of the brachiocephalic veins Venous tributaries The brachiocephalic veins drain venous blood from the head and neck, upper limbs and thoracic wall by receiving the following tributaries: On both sides, the brachiocephalic veins receive blood from the internal jugular, subclavian, vertebral, internal thoracic, inferior thyroid, first posterior intercostal and thymic veins. In addition, on the left side, the left superior intercostal vein and pericardial veins drain to the left brachiocephalic vein. 3

5 Lymphatic tributaries The large lymphatic trunks at the root of the neck usually open at the venous angle between the internal jugular and subclavian veins as follows: The right jugular, subclavian and bronchomediastinal lymphatic trunks open separately in the right venous angle or unite into a short right lymphatic duct. The thoracic duct opens into the left venous angle after receiving the left jugular, subclavian and bronchomediastinal lymphatic trunks. The Phrenic Nerves Each phrenic nerve is formed in the neck by the ventral rami of C3, C4 and C5. The nerves descend in the neck on the fascia of scalenus anterior muscle to reach the superior thoracic aperture where they enter the thorax posterior to the corresponding 1 st costal cartilage. The right phrenic nerve crosses anterior to the right subclavian artery and descends to the right of the right brachiocephalic vein and then the superior vena cava. It crosses anterior to the root of the lung and continues on the right side of the pericardium over the right atrium to reach the right side of the inferior vena cava with which it leaves the thorax through the caval opening of the diaphragm. The left phrenic nerve crosses the left subclavian artery and continues to the left of the aortic arch. It crosses anterior to the root of the lung and descends on the left side of the pericardium over the left atrium and left ventricle to leave the thorax by piercing the left dome of the diaphragm. Branches The phrenic nerves give two types of somatic branches; Motor branches supply the diaphragm and are most numerous on the inferior surface of the diaphragm. Sensory branches carry general sensory stimuli from the fibrous and parietal parts of the pericardium, the mediastinal and central diaphragmatic parts of the parietal pleura and from the central diaphragmatic part of the parietal peritoneum. The phrenic nerves and referred pain Because the phrenic nerves carry sensory impulses from a wide area of serous membranes (pericardium, pleura and peritoneum), pain from inflammation or irritation of these membranes is usually felt at the dermatomes that correspond to the root value of the phrenic nerves (C3-C5). This phenomenon is known as referred pain. Most commonly, inflammation of the gallbladder spreads to the central diaphragmatic part of the peritoneum and the resulting pain is referred by the right phrenic nerve to the right shoulder. The Thoracic Aorta The thoracic part of the aorta can be divided into three parts [Figure 21]; ascending aorta, arch of aorta, and descending aorta. The ascending part lies in the middle mediastinum, the arch is in the superior mediastinum and the descending part lies in the posterior mediastinum. The ascending aorta The ascending aorta is about 5 cm long and lies within the pericardial sac. It projects from the left ventricle upwards and to the right to the level of the sternal angle where the aortic arch begins. At its origin, the wall 4

6 of the ascending aorta shows three hemispherical outward bulges called the aortic sinuses which correspond to the cusps of the aortic valve. They are posterior, left and right in position. The right and left coronary arteries which supply the heart, arise from the right and left sinuses, respectively. The arch of the aorta The aortic arch begins as the continuation of the ascending aorta at the level of the sternal angle behind the right 1 / 2 of the sternum and ascends backwards and to the left. It reaches its highest level posterior to the midpoint of the manubrium and anterior to the trachea. Then it passes backwards and to the left to end at the left side of the disc between T4 and T5 vertebra. Here the descending aorta begins. As the arch of the aorta makes its curve, the following structures lie within its concavity: The pulmonary trunk. The Left main bronchus. The right pulmonary artery. The ligamentum arteriosum. The left recurrent laryngeal nerve; curves beneath the aortic arch just deep to the ligamentum arteriosum. The aortic bodies; are collections of nerve cells that act as chemoreceptors concerned with respiratory reflexes. The cardiac and pulmonary nerve plexuses; lie in and near the concavity of the arch. The aortic arch gives three branches that arise from its convexity (i.e. superior surface). From proximal to distal, they are: The brachiocephalic trunk is the largest of the three branches of the aortic arch. At its point of origin behind the center of the manubrium, it is slightly anterior to the other two branches. It ascends obliquely upwards backwards and to the right behind the right brachiocephalic vein. It ends posterior to the right sternoclavicular joint by dividing into right common carotid and right subclavian arteries which supply the right side of the head and neck and the right upper limb, respectively. Occasionally, the brachiocephalic trunk gives a small branch, the thyroidea ima artery, which contributes to the arterial supply of the thyroid gland. The left common carotid artery arises from the aortic arch immediately to the left and slightly posterior to the brachiocephalic trunk and ascends through the superior mediastinum along the left side of the trachea. It supplies the left side of the head and neck. The left subclavian artery arises from the arch of aorta immediately to the left of, and slightly posterior to, the left common carotid artery and ascends through the superior mediastinum along the left side of the trachea. It leaves the thorax by curving on the superior surface of the first rib, posterior to the insertion of scalenus anterior muscle. It suplies the left upper limb. The descending aorta The descending aorta begins as the continuation of the aortic arch on the left side of the 4 th thoracic intervertebral disc. It descends to the left of the vertebral bodies posterior to the root of the left lung, the pericardium and lower part of the esophagus. As it crosses behind the esophagus, it deviates towards the midline to become anterior to the lower thoracic vertebrae. It leaves the thorax by passing through the aortic 5

7 hiatus of the diaphragm deep to the median arcuate ligament at the level of the lower border of T12 vertebra where it becomes the abdominal aorta. Figure 4. The parts and branches of the thoracic aorta, numbers indicate the posterior intercostal arteries. Branches of the descending aorta Pericardial branches are few small vessels to the posterior surface of the pericardial sac. Two left bronchial arteries. Esophageal branches are four or five vessels which form a continuous anastomotic chain around the esophagus with esophageal branches of the inferior thyroid artery superiorly and esophageal branches of the left gastric artery inferiorly. Mediastinal branches are several small branches that supply lymph nodes, vessels, nerves and areolar tissue in the posterior mediastinum. Nine pairs of posterior intercostal arteries to the 3 rd -11 th intercostal spaces. One pair of subcostal arteries. Superior phrenic arteries are small vessels from the lower part of the descending aorta that supply the posterior part of the upper surface of the diaphragm. 6

8 The Vagus Nerves The vagus nerve is the 10 th cranial nerve. It provides parasympathetic innervation to the thoracic and abdominal viscera and carries visceral afferent information from these viscera. Visceral afferents in the vagus nerve relay information to the central nervous system about normal physiologic processes and reflex activities. They do not transmit pain sensation. After running its course in the head and neck, each vagus nerve enters the thorax posterior to the corresponding sternoclavicular joint and runs through the superior and posterior parts of the mediastinum. The vagus nerves leave the thorax with the esophagus through the esophageal hiatus of the diaphragm. The right vagus descends between the right brachiocephalic vein anteriorly, the brachiocephalic trunk posteriorly, the azygos arch laterally and the trachea medially. It continues posterior to the lung root to reach the esophagus. The left vagus descends between the left brachiocephalic vein anteriorly and the left common carotid and left subclavian arteries posteriorly. It then crosses the left side of the arch of aorta and runs posterior to the root of the left lung to reach the esophagus in the posterior mediastinum. As the two vagi reach the anterior surface of the esophagus they break up into the esophageal plexus. Figure 5. The trachea, esophagus and vagi, anterior view. Branches of the vagus nerves in the thorax Pulmonary, esophageal and cardiac branches arise from each vagus nerve as it crosses posterior to the lung root to supply the corresponding plexuses. 7

9 The left recurrent laryngeal nerve arises from the left vagus as it crosses the left side of the aortic arch. It curves below the aortic arch just deep to the ligamentum arteriosum and ascends medial to the aortic arch. Entering a groove between the trachea and esophagus, the left recurrent laryngeal nerve continues superiorly to enter the neck and terminate in the larynx. The right recurrent laryngeal nerve arises from the right vagus at the root of the neck, not in the thorax. The Trachea The trachea is a patent mobile fibrocartilaginous tube measuring cm in length and 2.5 cm in diameter. Half of it lies in the neck and half in the superior mediastinum. The trachea begins at the level of the body of C6 vertebra, enters the thorax posterior to the suprasternal notch and ends at the level of the sternal angle by dividing into right and left main bronchi. It runs vertically in the midline except for its lower part which is slightly pushed to the right by the aortic arch. The trachea is composed of C-shaped cartilaginous rings with the defect of the C directed posteriorly, this defect is bridged in each cartilage by the trachealis muscle. Relations in the thorax Anteriorly: the aortic arch, the brachiocephalic trunk, left common carotid artery, left brachiocephalic vein and thymus. To the left: the distal end of the aortic arch, left subclavian artery and the left vagus and left phrenic nerves. Posteriorly: the esophagus with the left recurrent laryngeal nerve To the right: the right vagus nerve descends vertically while the azygos arch crosses from back to front towards the superior vena cava. Blood supply, lymphatic drainage and innervation The arterial supply is derived from the inferior thyroid artery in the neck and from the bronchial arteries in the thorax. The corresponding veins drain to the brachiocephalic veins. Lymphatic drainage passes to the tracheobronchial, paratracheal and pretracheal lymph nodes. Visceral parasympathetic efferent nerve fibers are provided by the vagus and recurrent laryngeal nerves. Sympathetic postganglionic fibers arise from the sympathetic trunk. The Esophagus The esophagus is a 25 cm long muscular tube that extends from the neck to the abdomen. Unlike the trachea, the esophageal walls are kept in opposition and open only during swallowing. The esophagus begins at the lower end of the pharynx opposite the body of C6 vertebra, traverses the superior and posterior parts of the mediastinum and leaves the thorax through the esophageal hiatus opposite T10 vertebra. It runs obliquely to the left for 1-2 cm in the abdomen and enters the gastric cardia opposite the body of T11 vertebra. In the thorax, the esophagus descends on the anterior aspect of the bodies of the vertebrae in a midline position. As it approaches the diaphragm, it moves anteriorly and to the left, crossing from the right side of the thoracic aorta to become anterior to it. Important Relations in the thorax Anteriorly: the trachea and recurrent laryngeal nerves, the left main bronchus, the right pulmonary artery, the left atrium and the anterior vagal trunk. Posteriorly: the vertebral column and prevertebral muscles. The esophagus is crossed posteriorly at its lower part by the descending aorta and the posterior vagal trunk. At its upper part it is crossed posteriorly by the hemiazygos veins, the thoracic duct. 8

10 Esophageal constrictions The esophagus is a flexible, muscular tube that is normally compressed or narrowed by surrounding structures at four locations; These constrictions have important clinical consequences. For example, a swallowed object is most likely to lodge at a constricted area. An ingested corrosive substance would move more slowly through a narrowed region, causing more damage at this site than elsewhere along the esophagus. Also, constrictions present areas of resistance during the passage of instruments. Blood supply and lymphatic drainage Being a long muscular tube that traverses three body regions, the esophagus is supplied and drained by an anastomotic chain of blood and lymphatic vessels derived from vessels in the neck, thorax and abdomen. The cervical part is supplied by branches of the inferior thyroid artery and drained by the brachiocephalic veins. Its lymph passes to the deep cervical lymph nodes. The thoracic part receives arterial branches from the descending aorta and bronchial arteries and is drained by the azygos venous system. The lymph passes to the tracheobronchial and posterior mediastinal nodes. The abdominal (and lower most thoracic) part is supplied by the left gastric artery and drained by the left gastric vein. The lymph passes to the left gastric and celiac lymph nodes. Innervation The muscular layer of the upper part of the esophagus consists of striated muscles that are supplied by somatic fibers from the vagus and recurrent laryngeal nerves. The rest of the esophageal muscles are smooth fibers that receive parasympathetic fibers from the vagus nerves and sympathetic fibers from the sympathetic trunk. The parasympathetic fibers stimulate muscular contraction and mucus secretion. Esophageal branches of the vagus nerves and the thoracic sympathetic ganglia form the esophageal plexus around the lower thoracic part of the esophagus. There is some mixing of fibers from the two vagus nerves in 9

11 the esophageal plexus. Just above the diaphragm, fibers of the plexus converge to form two trunks; the anterior vagal trunk, mainly from fibers originally in the left vagus nerve; and the posterior vagal trunk mainly from fibers originally in the right vagus nerve. The vagal trunks continue on the corresponding surfaces of the esophagus as it passes through the diaphragm into the abdomen. The Thoracic Duct and Thoracic Lymph Nodes The thoracic duct is the principal channel through which lymph from most of the body is returned to the venous system. The duct contains many valves along its course. The lymph in the duct is characteristically white due to its rich content of absorbed fats from the gut. Half of the lymph is said to come from the liver. The thoracic duct begins in the abdomen as a confluence of lymph trunks that form a saccular dilation called the cisterna chyli. The cisterna chyli (5cm x 4mm) lies on the upper two lumbar vertebrae between the aorta and the azygos vein, hidden by the right crus of the diaphragm. Figure 6. The thoracic duct and azygos venous system. Numbers refer to the posterior intercostal veins. Course The thoracic duct ascends from the cisterna chyli posterior to the aorta and enters the thorax through the aortic hiatus of the diaphragm. In the posterior mediastinum, the duct ascends to the right of midline between the descending aorta on the left, the azygos vein on the right, the esophagus anteriorly and vertebral bodies posteriorly. At the level of T5 vertebra, it crosses obliquely behind the esophagus from right to left and enters the superior mediastinum. At the root of the neck, it opens into the venous angle between the left internal jugular and left subclavian veins. 10

12 Tributaries Lymphatics afferent vessels either drain to the thoracic duct directly or to the cisterna chyli first. The cisterna chyli receives the following afferent vessels: The right and left lumbar trunks; drain the lower limbs, the lower part of the anterior abdominal wall, the pelvis and the entire posterior abdominal wall including the gonads, kidneys and suprarenal glands. The intestinal lymph trunks; drain the gastrointestinal organs of the abdomen except the upper part of the liver which drains directly through the diaphragm to the mediastinal or parasternal lymph nodes. The lower posterior intercostal lymphatic vessels; accompany the lower posterior intercostal vessels. The thoracic duct receives the following afferent vessels: The middle and upper posterior intercostal lymph vessels. Lymph vessels from the esophagus and posterior parts of the diaphragm and pericardium. The left bronchomediastinal trunk. The left jugular trunk which drains the left side of the head and neck. The left subclavian trunk which drains the left upper limb. Major thoracic lymph nodes The parasternal nodes lie along the internal thoracic vessels and drain the area supplied by these vessels and the upper part of the liver. The intercostal nodes lie along the posterior intercostal vessels at the rib angles and drain lymph from the intercostal spaces to the cisterna chyli and thoracic duct. The posterior mediastinal nodes lie along the descending aorta and drain the posterior mediastinum including the diaphragm, pericardium and esophagus. The anterior mediastinal nodes lie along the left brachiocephalic vein and drain the heart, pericardium and thymus. The bronchopulmonary nodes lie in the lung hilum and the angles of bifurcation of large bronchi and drain the lung and visceral pleura. The tracheobronchial nodes lie on the thoracic trachea and main bronchi. The paratracheal nodes lie on each side of the trachea. The Azygos Venous System The azygos system of veins consists of longitudinal venous channels on each side of the vertebral column in the posterior and superior mediastinum. These veins communicate with each other and drain venous blood from the thoracic wall and some thoracic viscera. The system provides an alternative pathway for blood to return to the heart via the superior vena cava if the inferior vena cava is blocked. The three main veins of the azygos system are the azygos, hemiazygos and accessory hemiazygos veins. The hemiazygos and accessory hemiazygos veins are sometimes called the inferior and superior hemiazygos veins, respectively. The azygos vein The azygos vein arises opposite L1 or L2 vertebra at the junction between the right ascending lumbar vein and the right subcostal vein. It may also arise as a direct branch of the inferior vena cava, which is joined by a common trunk from the junction of the right ascending lumbar vein and the right subcostal vein. The azygos vein enters the thorax through the aortic hiatus of the diaphragm, or posterior to the right crus of the diaphragm. It ascends through the posterior mediastinum to the right of the thoracic duct. At the level the sternal angle, it arches anteriorly over the root of the right lung to join the superior vena cava just before the superior vena cava enters the pericardial sac. 11

13 Tributaries of the azygos vein include; The right superior intercostal vein, a single vessel formed by the junction of the 2 nd, 3 rd and 4 th posterior intercostal veins. The 5 th to the 11 th right posterior intercostal veins and the right subcostal vein. The hemiazygos and accessory hemiazygos veins. Esophageal, mediastinal, pericardial and right bronchial veins. The hemiazygos vein The hemiazygos vein usually arises at the junction between the left ascending lumbar vein and the left subcostal vein. It may also arise from either of these veins alone and often has a connection to the left renal vein. It usually enters the thorax through the left crus of the diaphragm, but may enter through the aortic hiatus. It ascends through the posterior mediastinum, on the left side of the midline, to the level of T9 vertebra. At this point, it crosses the vertebral column, posterior to the aorta, esophagus, and thoracic duct, to enter the azygos vein. Tributaries joining the hemiazygos vein include; The 9 th -11 th posterior intercostal veins and the left subcostal vein. Esophageal and mediastinal veins. The accessory hemiazygos vein The accessory (superior) hemiazygos vein descends on the left side from the superior portion of the posterior mediastinum to the level of T8 vertebra where it crosses the vertebral column to join the azygos vein, or ends in the hemiazygos vein, or has a connection to both veins. Usually, it also has a connection superiorly to the left superior intercostal vein. The accessory hemiazygos vein receives the 5 th -8 th left posterior intercostal veins and the left bronchial veins. THE PERICARDIUM The pericardium is a fibroserous sac that encloses the heart and the roots of the great vessels in the middle mediastinum. It is composed of outer fibrous and inner serous parts. The fibrous pericardium is composed of dense connective tissue. Inferiorly, it closely adheres to the central tendon of the diaphragm which separates it from the left lobe of the liver and the gastric fundus. Laterally, it adheres to the mediastinal pleura except where the two are separated by the phrenic nerves. Posteriorly, the fibrous pericardium blends with the adventitia of the great vessels at the base of the heart. Anteriorly, the fibrous pericardium is attached to the posterior surface of the sternum by two condensations of fibrous tissue called the superior and inferior sternopericardial ligaments. With its attachments and adherence to surrounding structures, the fibrous pericardium limits excessive movement of the beating heart. The serous pericardium is a closed sac that is invaginated by the heart from above and behind. Like the pleura, it is composed of parietal and visceral layers with a narrow pericardial cavity filled with some pericardial fluid in between. The parietal pericardium lines the fibrous pericardium. The visceral pericardium or epicardium covers the heart and reflects upon the great vessels at the base of the heart to become continuous with the parietal pericardium. 12

14 Figure 7. Sagittal section through the pericardial sac showing its layers and sinuses. The Pericardial Sinuses A pericardial sinus is a relatively wide area of pericardial cavity bounded by reflections of serous pericardium and filled with pericardial fluid. The visceral layer of the serous pericardium is continuous with the parietal layer around the roots of the great vessels. The reflections of serous pericardium occur in two locations; one superiorly, in relation to the arteries and the other more posteriorly, in relation to the veins. The transverse and oblique pericardial sinuses are limited by these reflections. The transverse pericardial sinus is a passageway in the serous pericardium that lies between the pulmonary trunk and aorta above; and the right and left superior pulmonary veins and superior vena cava below. The oblique pericardial sinus is an inverted U-shaped blind pocket of the pericardial cavity created as the serous pericardium reflects from the right over the superior and inferior venae cavae and the right pulmonary veins, from the left over the left pulmonary veins and from above over the pulmonary trunk. The sinus lies posterior to the left atrium and separates it from the esophagus. When the pericardium is opened anteriorly during surgery, a finger placed in the transverse sinus separates arteries from veins. A hand placed under the apex of the heart and moved superiorly slips into the oblique sinus. Blood Supply, Lymphatic Drainage and Innervation The pericardium is supplied by arterial branches from the internal thoracic, pericardiophrenic, musculophrenic, inferior phrenic arteries and the thoracic aorta. Veins from the pericardium enter the azygos venous system and the internal thoracic vein. Lymphatic vessels of the pericardium drain to the anterior and posterior mediastinal lymph nodes. 13

15 The fibrous pericardium and parietal layer of the serous pericardium receive somatic sensory nerve supply from the phrenic nerves. The visceral pericardium receives sympathetic fibers from T1 and T2 spinal segments and parasympathetic fibers from the vagus nerve; all through the cardiac plexus. Figure 8. The serous pericardial sac after the heart has been removed, anterior view. THE HEART General Description and Function The heart is a hollow muscular organ that consists of four chambers; right and left atria separated by the interatrial septum, and right and left ventricles separated by the interventricular septum. Each atrium pumps blood to the ventricle on the same side via the atrioventricular orifice guarded by an atrioventricular valve. Each ventricle pumps blood to a major artery. Blood flow always occurs in one direction, from the atria to the ventricles and from the ventricles to the arteries. Retrograde blood flow is prevented by the presence of heart valves at the atrioventricular orifices and at the roots of the major arteries. The function of the heart is to pump blood and maintain blood circulation [Figure 27].The blood circulation begins with the return of venous blood to the right atrium via the superior and inferior venae cavae and the coronary sinus. Blood from the right atrium is pumped through the right atrioventricular orifice to the right ventricle. From the right ventricle, the venous blood passes through the pulmonary trunk to the lungs to be oxygenated. The oxygenated blood returns from the lungs to the left atrium via the four pulmonary veins. The left atrium pumps this blood to the left ventricle through the left atrioventricular orifice. From the left ventricle, blood is ejected into the aorta to be distributed to all body tissues. After gaseous and nutrient exchange at the tissues, the venous blood is returned to the right atrium and the cycle completes. The passage of blood from the right ventricle to the lungs and its return to the left atrium is the pulmonary circulation. The pumping of blood from the left ventricle to the body tissues and its return to the right atrium is the systemic circulation. 14

16 Whenever a cardiac chamber fills with blood it is in a state of relaxation or diastole and when it contracts to pump the blood it is said to be in systole. The cardiac cycle is organized so that the atria contract together while the ventricles are relaxed and the ventricles contract together when the atria are relaxed. Size, Position and Orientation Figure 9. The systemic & pulmonary circulations. The heart is frequently described as having the size of the person s clenched fist. Its average weight in the adult is variable but is around g. The position of the heart in the thoracic cavity depends on body posture, the position of the diaphragm and the state of respiration. In the erect position the heart lies at the level of T7-T10 vertebrae at the end of inspiration, but rises to the level of T5-T8 vertebrae at the end of expiration. The general shape and orientation of the heart is that of a four sided pyramid that has fallen over and is resting on one of its sides so that the apex of the pyramid projects anteroinferiorly and to the left whereas the base faces posteriorly. The four sides of the pyramid are anterior, inferior, right and left. The heart, like this pyramid, has an apex, base, anterior (sternocostal) surface and inferior (diaphragmatic) surface. The right and left (pulmonary) surfaces are narrow and represent the right and left borders of the heart. The anterior surface of the heart is quadrilateral and has four borders; superior, inferior, right and left. Because the great vessels attach to the base of the heart, the base is fixed posteriorly to the pericardial sac. From the base the heart projects along its longitudinal axis forward, downward, and to the left, ending in the apex. The apex of the heart is formed by the inferolateral part of the left ventricle and is positioned at the left 5 th intercostal space just medial to the midclavicular line, which is about 8-9 cm from the midline. This 15

17 orientation of the heart brings the atria, which form the base of the heart, posterior to the ventricles. At the same time, the right chambers contribute more to the front of the heart while the left chambers contribute to the back. The planes of the atrioventricular orifices are more vertical than horizontal; hence the blood flows almost horizontally forward from the atria to the ventricles. The Cardiac Borders The cardiac borders represent the four borders of its anterior surface that can be seen on a chest X-ray and can also be traced on the surface of the thoracic cage. The superior border is slightly oblique from left to right. It begins in the upper part of the left 2 nd intercostal space; 1-2 cm from the sternal margin, and ends in the lower part of the right 2 nd intercostal space; 1-2 cm from the sternal margin. It is formed by the right and left atria under cover of the pulmonary trunk and ascending aorta. The right border passes from the right 2 nd intercostal space to the right 5 th intercostal space; 1-2 cm from the sternal margin. It is slightly convex to the right and is formed by the superior vena cava, right atrium and inferior vena cava, in that order from above downwards. The inferior border passes from the right 5 th intercostal space to a point in the left 5 th intercostal space just medial to the midclavicular line i.e. the point of the cardiac apex. It is slightly convex upwards and is formed by the right ventricle and a small part of the left ventricle. The left border extends obliquely from the cardiac apex to the left 2 nd intercostal space; 1-2 cm from the sternal margin. It is formed by the left ventricle and the left auricle above. This roughly quadrangular area demarcated by these borders is dull to percussion and clinicians refer to it as the precordium. Figure 10. The borders of the heart, anterior view. 16

18 The Surface Sulci The partitions that divide the heart into chambers are the interatrial and interventricular septa and the fibrous margins of the atrioventricular orifices. The fibrous margins of the atrioventricular orifices and the interventricular septum produce external grooves on the surface of the heart called sulci. The coronary (atrioventricular) sulcus is produced by the fibrous tissue of the atrioventricular orifices. It encircles the heart, separating the atria from the ventricles. Because of the orientation of the heart, it runs almost vertically on the anterior surface but becomes more horizontal posteriorly. The horizontal part of the coronary sulcus separates the base of the heart from the inferior surface. The anterior and posterior interventricular sulci are produced by the interventricular septum. The anterior interventricular sulcus separates the two ventricles on the anterior surface of the heart. The posterior interventricular sulcus separates the two ventricles on the inferior (diaphragmatic) surface of the heart. The two sulci are continuous with each other below the cardiac apex. The cardiac sulci contain the coronary arteries and their branches; and the cardiac veins and their tributaries. These blood vessels are normally buried in collections of fatty tissue. The Cardiac Surfaces The anterior or sternocostal surface is formed mainly by the right ventricle with small part of the right atrium on the right and small part of the left ventricle on the left. The right atrium is separated from the right ventricle by the anterior (vertical) part of the coronary sulcus containing the right coronary artery. The right ventricle is separated from the left ventricle by the anterior interventricular sulcus which contains the anterior interventricular artery and the great cardiac vein. The inferior or diaphragmatic surface is the surface on which the heart rests on the central tendon of the diaphragm. It is formed mainly by the left ventricle with small part of the right ventricle to the right. The two ventricles are separated from each other by the posterior interventricular sulcus which contains the posterior interventricular artery and the middle cardiac vein. The base of the heart represents its posterior or vertebral surface. It is formed mainly by the left atrium with small part of the right atrium. The atria in the base are separated from the ventricles in the inferior surface by the posterior (horizontal) part of the coronary sulcus which contains the circumflex and right coronary arteries, the coronary sinus and the continuations of the small and great cardiac veins. 17

19 Figure 11. The surfaces of the heart in anterior (above) and posteroinferior views (below). 18

20 The Cardiac Chambers and Septa The right atrium Three veins drain blood into the right atrium. The superior vena cava drains venous blood from the head, neck, upper limbs and thoracic wall. The inferior vena cava drains the rest of the body. The coronary sinus drains blood from the heart itself. The superior vena cava enters the upper posterior portion of the right atrium, and the inferior vena cava and coronary sinus enter the lower posterior portion of the right atrium. From the right atrium, blood passes into the right ventricle through the right atrioventricular orifice. This opening faces forward and is guarded by the tricuspid valve. Figure 12. The internal features of the right atrium and right ventricle. Externally, the right atrium is composed of two parts; the auricle and the venous part. The auricle is the small earlike, conical muscular pouch that extends anteriorly from the right atrium and overlaps the ascending aorta. The venous part is directed posteriorly and receives the veins that drain into the right atrium. The two parts are separated from each other by a groove called the sulcus terminalis. On the internal surface [Figure 30], the sulcus terminalis produces an elevated ridge called the crista terminalis. The crista begins on the roof of the atrium just in front of the opening of the superior vena cava and extends down the interatrial septum to end between the opening of the inferior vena cava and coronary sinus. The part of the atrial cavity posterior to the crista is called the sinus venarum. It has smooth, thin walls and both venae cavae empty into it. The part of the atrial cavity anterior to the crista, including the right auricle, is sometimes referred to as the atrium proper. Its walls are covered by muscular ridges called the musculi pectinati (pectinate muscles), which fan out from the crista like the teeth of a comb. 19

21 The fetal circulation During fetal life, the interatrial septum develops from two incompletely overlapping septa called the septum primum and septum secundum [Figure 31]. The foramen ovale represents an oval defect between the two septa. The fetal blood returning to the right atrium from the placenta is rich with oxygen and has to pass to the left side of the heart without passing to the fetal lungs; which are not functional. The foramen ovale allows passage of most of the right atrial blood to the left atrium. The remaining amount of blood that passes to the right ventricle is pumped through the pulmonary trunk and from there it passes via the ductus arteriosus to the aorta. As a result, all the oxygenated blood that returns to the fetal right atrium is transferred to the arterial side of the circulation. After birth and due to initiation of breathing, the blood pressure on the left side of the heart exceeds the pressure in the right side causing the septum primum and septum secundum to adhere closely to each other and fuse; so that the foramen ovale is closed. The remnant of the foramen ovale and septum primum form an oval depression called the fossa ovalis. The sharp lower margin of the septum secundum forms the margin of this fossa; the limbus. At the same time, the ductus arteriosus obliterates and the remnant fibrous band forms the ligamentum arteriosum. Failure of closure of the fossa ovalis results in an atrial septal defect. Failure of obliteration of the ductus arteriosus results in a condition known as patent ductus arteriosus. The interatrial septum This septum separates the right atrium from the left atrium. It faces forward and to the right because the left atrium lies posteriorly and to the left of the right atrium. A depression with a sharp margin is clearly visible in the septum just above the orifice of the inferior vena cava. This is the fossa ovalis (oval fossa), with its prominent margin, the limbus fossa ovalis. The fossa ovalis marks the location of the embryonic foramen ovale, which is an important part of the fetal circulation. Figure 13. The foramen ovale in the fetal heart. 20

22 The right ventricle The right ventricle is located anterior to the right atrioventricular orifice. It appears triangular in a longitudinal section [Figure 30] with an upper funnel-shaped outflow tract that leads to the pulmonary trunk. This outflow tract is called the infundibulum. The walls of the right ventricle are smooth in the infundibulum and near the right atrioventricular orifice. Elsewhere, the walls have numerous irregular muscular ridges called the trabeculae carneae. The trabeculae carneae of the right ventricle exist in either of three forms: Most trabeculae are either attached to the ventricular walls throughout their length, forming ridges, or attached at both ends across the ventricular cavity, forming bridges. A few trabeculae carneae form the papillary muscles which have only one end attached to the ventricular surface, while the other end serves as the point of attachment for tendon-like fibrous cords called the chordae tendineae. The chordae tendineae connect to the free edges of the cusps of the tricuspid valve. The papillary muscles; through the chordae tendineae, prevent the cusps of the tricuspid valve from flipping backwards into the right atrium during ventricular systole. That way they prevent retrograde flow of blood from the ventricle into the atrium. There are three papillary muscles in the right ventricle. Named relative to their point of origin on the ventricular surface, they are the anterior, posterior, and septal papillary muscles. A single specialized trabeculum forms a bridge between the lower portion of the interventricular septum and the base of the anterior papillary muscle. This septomarginal trabeculum or moderator band carries a portion of the cardiac conduction system, the right bundle of the atrioventricular bundle, to the anterior wall of the right ventricle. The interventricular septum This septum separates the two ventricles and forms part of the wall of each. Due to heart rotation, the right surface of the interventricular septum faces anteriorly and to the right while the left surface faces posteriorly and to the left. The septum develops from two parts. The upper small part is thin and membranous. The lower part forms the majority of the septum. It is thick and muscular. Failure of union of the membranous and muscular parts results in a ventricular septal defect. Because left ventricular pressure is greater than the right ventricular pressure, the septum bulges to the right so that on a cross section the right ventricle appears crescent-shaped while the left ventricle appears circular. The left atrium Most of the left atrium is smooth walled. This part lies posteriorly and contains the openings of the four pulmonary veins. The left auricle is a small anterior part of the left atrium. It overlaps the root of the pulmonary trunk and its walls are rough due to presence of musculi pectinati. Unlike the right atrium, there is no clear sulcus or crista between the two parts of the left atrium. The left atrioventricular orifice leads from the left atrium to the left ventricle and is guarded by the mitral (bicuspid) valve. The left ventricle The left ventricle lies anterior to the left atrium. Its anterior rounded end forms the cardiac apex. The interventricular septum forms the anterior wall of the left ventricle and separates it from the right ventricle. The walls of the left ventricle are normally three times thicker than the walls of the right ventricle. The longitudinal section of the left ventricle is conical in shape with its outflow tract; the aortic vestibule, passing towards the ascending aorta. The walls of the left ventricle are smooth in the aortic vestibule and near the left atrioventricular orifice. Elsewhere, the walls are rough due to presence of the trabeculae carneae. The trabeculae carneae in the left ventricle are fine and delicate in contrast to those in the right ventricle. The general appearance of the trabeculae with muscular ridges and bridges is similar to that of the right ventricle. Papillary muscles, together with chordae tendineae, are also observed and their structure is as described for 21

23 the right ventricle. However, only two papillary muscles are found in the left ventricle; the anterior and posterior papillary muscles. The anterior papillary muscle is large and passes to the anterior cusp of the mitral valve. The posterior one is smaller and passes to the posterior cusp. The Cardiac Valves The tricuspid and mitral valves The tricuspid and mitral valves (the atrioventricular valves) consist of cusps that are fixed by their bases to the fibrous rings of the atrioventricular orifices. The free edges of the cusps are held by the chordae tendineae of the papillary muscles. During emptying of the atria, the cusps are open to allow flow of blood into the ventricles. During contraction of the ventricles, the cusps close the atrioventricular orifice while the papillary muscles and the chordae tendineae prevent separation of the valvular cusps. The tricuspid valve consists of three cusps named anterior, posterior and septal. The mitral or bicuspid valve is composed of two cusps named anterior and posterior. The naming of the cusps is according to the wall of the ventricle to which they are attached. The anterior cusps of both valves are usually the largest cusps. The pulmonary and aortic valves The pulmonary and aortic valves lie at the distal end of the outflow tracts of the right and left ventricles, respectively. The pulmonary valve lies between the infundibulum of the right ventricle and the root of the pulmonary trunk. The aortic valve lies between the aortic vestibule and the root of the ascending aorta. These valves are called semilunar valves because each consists of three semilunar cusps. Each cusp forms a pocket-like sinus or a dilatation in the wall of the root of the pulmonary trunk or aorta. The cusps of the pulmonary valve are named left, right and anterior. The cusps of the aortic valve are named right, left, and posterior. The right and left coronary arteries originate from the right and left aortic sinuses. Because of this, the posterior aortic sinus and cusp are sometimes referred to as the noncoronary sinus and cusp. Figure 14. Superior view of the heart valves in situ. The four corner insets show the features of each valve. Letters indicate the name of the cusps of each valve (A: anterior, P: posterior, S: septal, R: right, L: left) 22

24 The semilunar cusps are not secured by chordae tendineae or papillary muscles. Instead, the integrity of the semilunar valves depends on the recoil of blood at the end of ventricular systole. Each semilunar cusp has a free edge projecting upward into the lumen of the pulmonary trunk or aorta. The free superior edge of each cusp has a middle, thickened portion called the nodule of the semilunar cusp; and a thin lateral portion called the lunule of the semilunar cusp. After ventricular contraction, the recoil of blood fills these sinuses and forces the cusps closed in a Y-shaped pattern so that the three nodules contact each other at the center. Chest auscultation for heart sounds Auscultation for heart sounds enables the clinician to listen to the heart valves as they close, giving an idea about the cardiac cycle and heart rate and rhythm. Each heart valve sound is heard with maximum clarity at an area that corresponds to the direction of blood flow through that valve [Figure 33].Blood flows upwards and to the right through the aortic valve, upwards and to the left through the pulmonary valve and forwards downwards and to the left through the tricuspid and mitral valves. Therefore; The aortic valve is heard over the medial end of the right second intercostal space. The pulmonary valve is heard over the medial end of the left second intercostal space. The tricuspid valve is heard just to the left of the lower part of the sternum at the fifth intercostal space. The mitral valve is heard over the apex of the heart in the left fifth intercostal space at the midclavicular line. Figure 15. Surface anatomy of the heart valves. Arrows lead to surface areas of auscultation for each valve. (A: aortic, P: pulmonary,t: tricuspid,m: mitral). 23

25 Structural Layers of the Heart The wall of each heart chamber consists of three layers; The endocardium; is the thin internal layer composed of endothelium and subendothelial connective tissue that lines the heart and covers its valves. The myocardium is the thickest layer. It is composed of myocardial fibers arranged into superficial transverse and deep arch-shaped layers. In the ventricles, an additional middle layer of oblique fibers is present. The myocardium is thicker in the ventricles than in the atria. It is also thicker in the left chambers than in the corresponding right chambers. The epicardium; is the thin external layer formed by the visceral layer of serous pericardium. The Fibrous Skeleton of the Heart The cardiac skeleton is a composed of dense fibrous connective tissue in the form of four rings (anulus fibrosus) with interconnecting areas in a plane between the atria and the ventricles. The four rings of the cardiac skeleton surround the two atrioventricular orifices, the aortic orifice and opening of the pulmonary trunk [Figure 34]. The interconnecting areas include two trigones. The right fibrous trigone is a thickened area of connective tissue between the aortic ring and right atrioventricular ring. The left fibrous trigone is a thickened area of connective tissue between the aortic ring and the left atrioventricular ring. The membranous part of the interventricular septum descends as a small projection from the lower margin of the skeleton. The cardiac skeleton serves three main functions. It maintains the integrity of the openings it surrounds and provides points of attachment for the valvular cusps. It also acts as a dense connective tissue partition that electrically isolates the atria from the ventricles. The atrial myocardium is attached to the upper border of the rings, whereas the ventricular myocardium is attached to the lower border of the rings. Figure 16. The fibrous skeleton of the heart, posterior view. 24

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