The Mediastinum

The Mediastinum.

The mediastinum is a movable partition that extends superiorly to the thoracic outlet and the root of the neck and inferiorly to the diaphragm. It extends anteriorly to the sternum and posteriorly to the vertebral column. It contains the remains of the thymus, the heart and large blood vessels, the trachea and esophagus, the thoracic duct and lymph nodes, the vagus and phrenic nerves, and the sympathetic trunks..

The mediastinum is divided into superior and inferior mediastina by an imaginary plane passing from the sternal angle anteriorly to the lower border of the body of the fourth thoracic vertebra posteriorly. The inferior mediastinum is further subdivided into the middle mediastinum, which consists of the pericardium and heart; the anterior mediastinum, which is a space between the pericardium and the sternum; and the posterior mediastinum, which lies between the pericardium and the vertebral column..

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Superior Mediastinum. The superior mediastinum is bounded in front by the manubrium sterni and behind by the first four thoracic vertebrae. (a) Thymus, (b) large veins, (c) large arteries, (d) trachea, (e) oesophagus and thoracic duct, cardiac plexuses and (f) sympathetic trunks Inferior Mediastinum The inferior mediastinum is bounded in front by the body of the sternum and behind by the lower eight thoracic vertebrae. (a) Thymus, (b) heart within the pericardium with the phrenic nerves on each side, (c) esophagus and thoracic duct, (d) descending aorta, and (e) sympathetic trunks.

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Diagram Description automatically generated.

The Heart & Pericardium.

The Heart & Pericardium. The pericardium is a fibroserous sac that encloses the heart and the roots of the great vessels. Tts function is to restrict excessive movements of the heart as a whole and to serve as a lubricated container in which the different parts of the heart can contract. The pericardium lies within the middle mediastinum, posterior to the body of the sternum and the second to the sixth costal cartilages and anterior to the fifth to the eighth thoracic vertebrae..

Fibrous Pericardium The fibrous pericardium is the strong fibrous part of the sac. It is firmly attached below to the central tendon of the diaphragm. It fuses with the outer coats of the great blood vessels passing through it. The fibrous pericardium is attached in front to the sternum by the sternopericardial ligaments. Serous Pericardium The serous pericardium lines the fibrous pericardium and coats the heart. It is divided into parietal and visceral layers. The parietal layer lines the fibrous pericardium and is reflected around the roots of the great vessels to become continuous with the visceral layer of serous pericardium that closely covers the heart.

The visceral layer is closely applied to the heart and is often called the epicardium. The slitlike space between the parietal and visceral layers is referred to as the pericardial cavity . Normally, the pericardial cavity contains a small amount of tissue fluid (about 50 mL), the pericardial fluid, which acts as a lubricant to facilitate movements of the heart..

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Pericardial Sinuses On the posterior surface of the heart, the reflection of the serous pericardium around the large veins forms a recess called the oblique sinus. Also on the posterior surface of the heart is the transverse sinus, which is a short passage that lies between the reflection of serous pericardium around the aorta and pulmonary trunk and the reflection around the large veins. The pericardial sinuses form as a consequence of the way the heart bends during development..

Pericardial Disease.

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Nerve Supply of the Pericardium The fibrous pericardium and the parietal layer of the serous pericardium are supplied by the phrenic nerves . The visceral layer of the serous pericardium is innervated by branches of the sympathetic trunks and the vagus nerves.

Clinical Notes. Pericarditis In inflammation of the serous pericardium, called pericarditis, pericardial fluid may accumulate excessively, which can compress the thin-walled atria and interfere with the filling of the heart during diastole. This compression of the heart is called cardiac tamponade . Cardiac tamponade can also occur secondary to stab or gunshot wounds when the chambers of the heart have been penetrated. The blood escapes into the pericardial cavity and can restrict the filling of the heart. Roughening of the visceral and parietal layers of serous pericardium by inflammatory exudate in acute pericarditis produces pericardial friction rub, which can be felt on palpation and heard through a stethoscope..

Pericardial fluid can be aspirated from the pericardial cavity should excessive amounts accumulate in pericarditis. This process is called paracentesis . The needle can be introduced to the left of the xiphoid process in an upward and backward direction at an angle of 45° to the skin. When paracentesis is performed at this site, the pleura and lung are not damaged because of the presence of the cardiac notch in this area..

The Heart. The heart is a hollow muscular organ that is somewhat pyramid shaped and lies within the pericardium in the mediastinum. It is connected at its base to the great blood vessels but otherwise lies free within the pericardium..

Surfaces of the Heart The heart has three surfaces, and It also has an apex , which is directed downward, forward, and to the left. It lies at the level of the fifth left intercostal space, 3.5 in. (9 cm) from the midline The sternocostal surface is formed mainly by the right atrium and the right ventricle, which are separated from each other by the vertical atrioventricular groove. The right ventricle is separated from the left ventricle by the anterior interventricular groove..

The diaphragmatic surface of the heart is formed mainly by the right and left ventricles separated by the posterior interventricular groove. The inferior surface of the right atrium, into which the inferior vena cava opens, also forms part of this surface. The base of the heart, or the posterior surface, is formed mainly by the left atrium, into which open the four pulmonary veins. The base of the heart lies opposite the apex..

Note that the base of the heart is called the base because the heart is pyramid shaped; the base lies opposite the apex. The heart does not rest on its base; it rests on its diaphragmatic (inferior) surface. Borders of the Heart The right border is formed by the right atrium. The left border, by the left auricle; and below, by the left ventricle. The lower border is formed mainly by the right ventricle but also by the right atrium; the apex is formed by the left ventricle. These borders are important to recognize when examining a radiograph of the heart..

Chambers of the Heart. The heart is divided by vertical septa into four chambers: the right and left atria and the right and left ventricles. The right atrium lies anterior to the left atrium, and the right ventricle lies anterior to the left ventricle. The walls of the heart are composed of cardiac muscle, the myocardium ; covered externally with serous pericardium, the epicardium ; and lined internally with a layer of endothelium, the endocardium ..

Right Atrium. The right atrium consists of a main cavity and a small outpouching, the auricle. On the outside of the heart at the junction between the right atrium and the right auricle is a vertical groove, the sulcus terminalis , which on the inside forms a ridge, the crista terminalis . The main part of the atrium that lies posterior to the ridge is smooth walled and is derived embryologically from the sinus venosus. The part of the atrium in front of the ridge is roughened or trabeculated by bundles of muscle fibers, the musculi pectinati , which run from the crista terminalis to the auricle. This anterior part is derived embryologically from the primitive atrium..

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Openings into the Right Atrium The superior vena cava opens into the upper part of the right atrium; it has no valve. The inferior vena cava (larger than the SVC) opens into the lower part of the right atrium; it is guarded by, nonfunctioning valve. The coronary sinus, which drains most of the blood from the heart wall, opens into the right atrium between the inferior vena cava and the atrioventricular orifice. It is guarded by, nonfunctioning valve. The right atrioventricular orifice lies anterior to the IVC opening and is guarded by the tricuspid valve. Many small orifices of small veins also drain the wall of the heart and open directly into the right atrium..

Fetal Remnants Fossa ovalis and anulus ovalis . These latter structures lie on the atrial septum, which separates the right atrium from the left atrium. The fossa ovalis is a shallow depression, which is the site of the foramen oval in the fetus. The anulus ovalis forms the upper margin of the fossa. The floor of the fossa represents the persistent septum primum of the heart of the embryo, and the anulus is formed from the lower edge of the septum secundum.

Right Ventricle. Communicates with the right atrium through the atrioventricular orifice and with the pulmonary trunk through the pulmonary orifice. As the cavity approaches the pulmonary orifice it becomes funnel shaped, at which point it is referred to as the infundibulum. The walls of the right ventricle are much thicker than those of the right atrium and show several internal projecting ridges formed of muscle bundles. These projecting ridges are known as trabeculae carneae . The trabeculae carneae are composed of three types..

* The first type comprises the papillary muscles , which project inward, being attached by their bases to the ventricular wall; their apices are connected by fibrous chords (the chordae tendineae ) to the cusps of the tricuspid valve..

Internal features of Heart Dr. Sama ul Haque Dr Rania Gabr. - ppt download.

The tricuspid valve guards the right atrioventricular orifice and consists of three cusps formed by a fold of endocardium with some connective tissue enclosed: anterior , septal , and inferior (posterior) cusps. The bases of the cusps are attached to the fibrous ring of the skeleton of the heart, whereas their free edges and ventricular surfaces are attached to the chordae tendineae. When the ventricle contracts, the papillary muscles contract and prevent the cusps from being forced into the atrium and turning inside out as the intraventricular pressure rises. To assist in this process, the chordae tendineae of one papillary muscle are connected to the adjacent parts of two cusps..

The pulmonary valve guards the pulmonary orifice and consists of three semilunar cusps formed by folds of endocardium with some connective tissue enclosed. The curved lower margins and sides of each cusp are attached to the arterial wall. The open mouths of the cusps are directed upward into the pulmonary trunk. The attachments of the sides of the cusps to the arterial wall prevent the cusps from prolapsing into the ventricle. At the root of the pulmonary trunk are three dilatations called the sinuses. The three semilunar cusps are arranged with one posterior (left cusp) and two anterior (anterior and right cusps)..

During ventricular systole, the cusps of the valve are pressed against the wall of the pulmonary trunk by the out-rushing blood. During diastole, blood flows back toward the heart and enters the sinuses; the valve cusps fill, come into apposition in the center of the lumen, and close the pulmonary orifice..

Pulmonic valve - Echocardiografie.nl. Types of Heart Valves | Aortic valve.

Left Atrium. Similar to the right atrium, the left atrium consists of a main cavity and a left auricle. The left atrium is situated behind the right atrium and forms the greater part of the base or the posterior surface of the heart. Behind it lies the oblique sinus of the serous pericardium, and the fibrous pericardium separates it from the esophagus. The interior of the left atrium is smooth, but the left auricle possesses muscular ridges as in the right auricle. Openings into the Left Atrium The four pulmonary veins, two from each lung, open through the posterior wall and have no valves. The left atrioventricular orifice is guarded by the mitral valve..

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Left Ventricle. The walls of the left ventricle are three times thicker than those of the right ventricle. (The left intraventricular blood pressure is six times higher than that inside the right ventricle.) In cross section, the left ventricle is circular; the right is crescentic because of the bulging of the ventricular septum into the cavity of the right ventricle. There are well-developed trabeculae carneae , two large papillary muscles, but no moderator band. The part of the ventricle below the aortic orifice is called the aortic vestibule..

View of the interior of the left ventricle, showing the mitral valve... | Download Scientific Diagram.

The mitral valve guards the atrioventricular orifice. It consists of two cusps, one anterior and one posterior. The anterior cusp is the larger and intervenes between the atrioventricular and the aortic orifices. The aortic valve guards the aortic orifice and is precisely similar in structure to the pulmonary valve. One cusp is situated on the anterior wall (right cusp) and two are located on the posterior wall (left and posterior cusps). Behind each cusp the aortic wall bulges to form an aortic sinus. The anterior aortic sinus gives origin to the right coronary artery, and the left posterior sinus gives origin to the left coronary artery..

Conducting System of the Heart. The normal heart contracts rhythmically at about 70 to 90 beats per minute in the resting adult. The rhythmic contractile process originates spontaneously in the conducting system and the impulse travels to different regions of the heart, so the atria contract first and together, to be followed later by the contractions of both ventricles together. The slight delay in the passage of the impulse from the atria to the ventricles allows time for the atria to empty their blood into the ventricles before the ventricles contract..

The conducting system of the heart consists of specialized cardiac muscle present in the sinuatrial node, the atrioventricular node, the atrioventricular bundle and its right and left terminal branches, and the subendocardial plexus of Purkinje fibers (specialized cardiac muscle fibers that form the conducting system of the heart)..

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Sinuatrial Node The sinuatrial node is located in the wall of the right atrium in the upper part of the sulcus terminalis just to the right of the opening of the superior vena cava. The node spontaneously gives origin to rhythmic electrical impulses that spread in all directions through the cardiac muscle of the atria and cause the muscle to contract. Atrioventricular Node The atrioventricular node is strategically placed on the lower part of the atrial septum just above the attachment of the septal cusp of the tricuspid valve. From it, the cardiac impulse is conducted to the ventricles by the atrioventricular bundle. The atrioventricular node is stimulated by the excitation wave as it passes through the atrial myocardium. The speed of conduction of the cardiac impulse through the atrioventricular node (about 0.11 seconds) allows sufficient time for the atria to empty their blood into the ventricles before the ventricles start to contract..

Atrioventricular Bundle The atrioventricular bundle (bundle of His) is the only pathway of cardiac muscle that connects the myocardium of the atria and the myocardium of the ventricles and is thus the only route along which the cardiac impulse can travel from the atria to the ventricles. The bundle descends through the fibrous skeleton of the heart. The atrioventricular bundle then descends behind the septal cusp of the tricuspid valve to reach the inferior border of the membranous part of the ventricular septum..

At the upper border of the muscular part of the septum it divides into two branches, one for each ventricle. The right bundle branch (RBB) passes down on the right side of the ventricular septum to reach the moderator band, where it crosses to the anterior wall of the right ventricle. Here it becomes continuous with the fibers of the Purkinje plexus. The left bundle branch (LBB) pierces the septum and passes down on its left side beneath the endocardium. It usually divides into two branches (anterior and posterior), which eventually become continuous with the fibers of the Purkinje plexus of the left ventricle..

It is thus seen that the conducting system of the heart is responsible not only for generating rhythmic cardiac impulses, but also for conducting these impulses rapidly throughout the myocardium of the heart so that the different chambers contract in a coordinated and efficient manner. The activities of the conducting system can be influenced by the autonomic nerve supply to the heart. The parasympathetic nerves slow the rhythm and diminish the rate of conduction of the impulse; the sympathetic nerves have the opposite effect..

Internodal Conduction Paths Impulses from the sinuatrial node have been shown to travel to the atrioventricular node more rapidly than they can travel by passing along the ordinary myocardium. This phenomenon has been explained by the description of special pathways in the atrial wall, which have a structure consisting of a mixture of Purkinje fibers and ordinary cardiac muscle cells. The anterior internodal pathway leaves the anterior end of the sinuatrial node and passes anterior to the superior vena caval opening. It descends on the atrial septum and ends in the atrioventricular node. The middle internodal pathway leaves the posterior end of the sinuatrial node and passes posterior to the superior vena caval opening. It descends on the atrial septum to the atrioventricular node. The posterior internodal pathway leaves the posterior part of the sinuatrial node and descends through the crista terminalis and the valve of the inferior vena cava to the atrioventricular node..

Arterial Supply of the Heart The arterial supply of the heart is provided by the right and left coronary arteries, which arise from the ascending aorta immediately above the aortic valve. The coronary arteries and their major branches are distributed over the surface of the heart, lying within subepicardial connective tissue. The right coronary artery arises from the anterior aortic sinus of the ascending aorta and runs forward between the pulmonary trunk and the right auricle. It descends almost vertically in the right atrioventricular groove, and at the inferior border of the heart it continues posteriorly along the atrioventricular groove to anastomose with the left coronary artery in the posterior interventricular groove. The following branches from the right coronary artery supply the right atrium and right ventricle and parts of the left atrium and left ventricle and the atrioventricular septum..

Branches The right conus artery supplies the anterior surface of the pulmonary conus (infundibulum of the right ventricle) and the upper part of the anterior wall of the right ventricle. The anterior ventricular branches are two or three in number and supply the anterior surface of the right ventricle. The marginal branch is the largest and runs along the lower margin of the costal surface to reach the apex. The posterior ventricular branches are usually two in number and supply the diaphragmatic surface of the right ventricle. The posterior interventricular (descending) artery runs toward the apex in the posterior interventricular groove. It gives off branches to the right and left ventricles, including its inferior wall..