Atlas of Anatomy

7 Mediastinum

Mediastinum: Overview

image The mediastinum is the space in the thorax between the pleural sacs of the lungs. It is divided into two parts: superior and inferior. The inferior mediastinum is further divided into anterior, middle, and posterior portions.

Fig. 7.1   Divisions of the mediastinum

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Fig. 7.2   Contents of the mediastinum

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Mediastinum: Structures

Fig. 7.3   Mediastinum

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Thymus & Pericardium

Fig. 7.4   Thymus and pericardium in situ

Anterior view of coronal section. The thymus lies in the superior mediastinum.

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Fig. 7.5   Thymus

Anterior view of opened thorax of a 2-year-old child. The thymus is well developed at this age, extending inferiorly into the anterior mediastinum (compare with Fig. 7.4). The thymus grows throughout childhood; at puberty, high levels of circulating sex hormones cause the thymus to atrophy.

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Fig. 7.6   Pericardium

Anterior view of opened thorax with flaps of fibrous pericardium reflected.

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Fig. 7.7   Serous pericardial reflections

Anterior view. The parietal and visceral serous pericardium are continuous with one another around the great vessels of the heart. The passage between the arterial- and venous-associated reflections is the transverse pericardial sinus (see B).

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Heart in Situ

image The heart is located posterior to the sternum in the middle portion of the inferior mediastinum. The heart projects into the left side of the thoracic cavity.

Fig. 7.8   Topographical relations of the heart

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Fig. 7.9   Circulation

Red: Oxygenated blood. Blue: Deoxygenated blood. See p. 94 for prenatal circulation.

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Fig. 7.10   Heart in situ

Anterior view.

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Heart: Surfaces & Chambers

image Note the reflection of visceral serous pericardium to become parietal serous pericardium.

Fig. 7.11   Surfaces of the heart

The heart has three surfaces: anterior (sternocostal), posterior (base), and inferior (diaphragmatic).

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Fig. 7.12   Chambers of the heart

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Heart: Valves

image The cardiac valves are divided into two groups: semilunar and atrioventricular. The two semilunar valves (aortic and pulmonary) located at the base of the two great arteries of the heart regulate passage of blood from the ventricles to the aorta and pulmonary trunk. The two atrioventricular valves (left and right) lie at the interface between the atria and ventricles.

Fig. 7.13   Cardiac valves

Plane of cardiac valves, superior view. Removed: Atria and great arteries.

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Fig. 7.14   Semilunar valves

Valves have been longitudinally sectioned and opened.

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Fig. 7.15   Atrioventricular valves

Anterior view during ventricular systole.

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Auscultation of the cardiac valves

Heart sounds, produced by closure of the semilunar and atrioventricular valves, are carried by the blood flowing through the valve. The resulting sounds are therefore best heard “downstream,” at defined auscultation sites (dark circles). Valvular heart disease causes turbulent blood flow through the valve; this produces a murmur that may be detected in the colored regions.

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Table 7.2 Position and auscultation sites of cardiac valves

Valve

Anatomical projection

Auscultation site

Aortic valve

Left sternal border (at level of 3rd rib)

Right 2nd intercostal space (at sternal margin)

Pulmonary valve

Left sternal border (at level of 3rd costal cartilage)

Left 2nd intercostal space (at sternal margin)

Left atrioventricular valve

Left 4th/5th costal cartilage

Left 5th intercostal space (at midclavicular line) or cardiac apex

Right atrioventricular valve

Sternum (at level of 3rd costal cartilage)

Left 5th intercostal space (at sternal margin)

Arteries & Veins of the Heart

Fig. 7.16   Coronary arteries and cardiac veins

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Fig. 7.17   Distribution of the coronary arteries

Anterior and posterior views of the heart, with superior views of transverse sections through the ventricles. The distribution of the coronary arteries differs from person to person. Right coronery artery and branches (green); left coronary artery and branches (red).

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Disturbed coronary blood flow

Although the coronary arteries are connected by structural anastomoses, they are end arteries from a functional standpoint. The most frequent cause of deficient blood flow is athero sclerosis, a narrowing of the coronary lumen due to plaque-like deposits on the vessel wall. When the decrease in luminal size (stenosis) reaches a critical point, coronary blood flow is restricted, causing chest pain (angina pectoris). Initially, this pain is induced by physical effort, but eventually it persists at rest, often radiating to characteristic sites (e.g., left arm, left side of head and neck). A myocardial infarction occurs when deficient blood supply causes myocardial tissue to die (necrosis). The location and extent of the infarction depends on the stenosed vessel (see A-E, after Heinecker).

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Conduction & Innervation of the Heart

image Contraction of cardiac muscle is modulated by the cardiac conduction system. This system of specialized myocardial cells generates and conducts excitatory impulses in the heart. The conduction system contains two nodes, both located in the atria: the sinoatrial (SA) node, known as the pacemaker, and the atrioventricular (AV) node.

Fig. 7.18   Cardiac conduction system

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Electrocardiogram (ECG)

The cardiac impulse (a physical dipole) travels across the heart and may be detected with electrodes. The use of three electrodes that separately record electrical activity of the heart along three axes or vectors (Einthoven limb leads) generates an electrocardiogram (ECG). The ECG graphs the cardiac cycle (“heartbeat”), reducing it to a series of waves, segments, and intervals. These ECG components can be used to determine whether cardiac impulses are normal or abnormal (e.g., myocardial infarction, chamber enlargement). Note: Although only three leads are required, a standard ECG examination includes at least two others (Goldberger, Wilson leads).

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image Sympathetic innervation: Presynaptic neurons from T1 to T6 spinal cord segments send fibers to synapse on postsynaptic neurons in the cervical and upper thoracic sympathetic ganglia. The three cervical cardiac nerves and thoracic cardiac branches contribute to the cardiac plexus. Parasympathetic innervation: Presynaptic neurons and fibers reach the heart via cardiac branches, some of which also arise in the cervical region. They synapse on postsynaptic neurons near the SA node and along the coronary arteries.

Fig. 7.19   Autonomic innervation of the heart

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Heart: Radiology

Fig. 7.20   Cardiac borders and configurations

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Table 7.5 Borders of the heart

Border

Defining structures

Right cardiac border

Right atrium

Superior vena cava

Apex

Left ventricle

Left cardiac border

Aortic arch (“aortic knob”)

 

Pulmonary trunk

 

Left atrium

 

Left ventricle

Inferior cardiac border

Left ventricle

 

Right ventricle

Fig. 7.21   Radiographic appearance of the heart

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Fig. 7.22   Heart in transverse section

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Pre- & Postnatal Circulation

Fig. 7.23   Prenatal circulation

After Fritsch and Kühnel.

1.     Oxygenated and nutrient-rich fetal blood from the placenta passes to the fetus via the umbilical vein.

2.     Approximately half of this blood bypasses the liver (via the ductus venosus) and enters the inferior vena cava. The remainder enters the portal vein to supply the liver with nutrients and oxygen.

3.     Blood entering the right atrium from the inferior vena cava bypasses the right ventricle (as the lungs are not yet functioning) to enter the left atrium via the foramen ovale, a right-to-left shunt.

4.     Blood from the superior vena cava enters the right atrium, passes to the right ventricle, and moves into the pulmonary trunk. Most of this blood enters the aorta via the ductus arteriosus, a right-to-left shunt.

5.     The partially oxygenated blood in the aorta returns to the placenta via the paired umbilical arteries that arise from the internal iliac arteries.

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1.     As pulmonary respiration begins at birth, pulmonary blood pressure falls, causing blood from the right pulmonary trunk to enter the pulmonary veins.

2.     The foramen ovale and ductus arteriosus close, eliminating the fetal right-to-left shunts. The pulmonary and systemic circulations in the heart are now separate.

3.     As the infant is separated from the placenta, the umbilical arteries occlude (except for the proximal portions), along with the umbilical vein and ductus venosus.

4.     Blood to be metabolized now passes through the liver.

Fig. 7.24   Postnatal circulation

After Fritsch and Kühnel.

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Septal defects

Septal defects, the most common type of congenital heart defect, allow blood from the left chambers of the heart to improperly pass into the right chambers during systole. Ventrical septal defect (VSD, shown below) is the most common form. Patent foramen ovale, the most prevalent form of atrial septal defect (ASD), results from improper closure of the fetal shunt.

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Table 7.6 Derivatives of fetal circulatory structures

Fetal structure

Adult remnant

Ductus arteriosus

Ligamentum arteriosum

Foramen ovale

Fossa ovalis

Ductus venosus

Ligamentum venosum

Umbilical v.

Round ligament of the liver (ligamentum teres)

Umbilical a.

Medial umbilical ligament

Esophagus

image The esophagus is divided into three parts: cervical (C6-T1), thoracic (T1 to the esophageal hiatus of the diaphragm), and abdominal (the diaphragm to the cardiac orifice of the stomach). It descends slightly to the right of the thoracic aorta and pierces the diaphragm slightly to the left, just below the xiphoid process of the sternum.

Fig. 7.25   Esophagus: Location and constrictions

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Fig. 7.26   Esophagus in situ

Anterior view.

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Fig. 7.27   Structure of the esophagus

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Esophageal diverticula

Diverticula (abnormal outpouchings or sacs) generally develop at weak spots in the esophageal wall. There are three main types of esophageal diverticula:

·   Hypopharyngeal (pharyngo-esophageal) diverticula: Outpouchings occurring at the junction of the pharynx and the esophagus. These include Zenker's diverticula (70% of cases).

·   “True” traction diverticula: Protrusion of all wall layers, not typicallyoccurring at characteristic weak spots. However, they generally result from an inflammatory process (e.g., lymphangitis) and are thus common at sites where the esophagus closely approaches the bronchi and bronchial lymph nodes (thoracic or parabronchial diverticula).

·   “False” pulsion diverticula: Herniations of the mucosa and submucosa through weak spots in the muscular coat due to a rise in esophageal pressure (e.g., during normal swallowing). These include parahiatal and epiphrenic diverticula occurring above the esophageal aperture of the diaphragm (10% of cases).

Neurovasculature of the Esophagus

image Sympathetic innervation: Presynaptic fibers arise from the T2-T6 spinal cord segments. Postsynaptic fibers arise from the sympathetic chain to join the esophageal plexus. Parasympathetic innervation: Presynaptic fibers arise from the dorsal vagal nucleus and travel in the vagus nerves to form the extensive esophageal plexus. Note: The postsynaptic neurons are in the wall of the esophagus. Fibers to the cervical portion of the esophagus travel in the recurrent laryngeal nerves.

Fig. 7.28   Autonomic innervation of the esophagus

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Fig. 7.29   Esophageal plexus

The left and right vagus nerves initially descend on the left and right sides of the esophagus. As they begin to contribute to the esophageal plexus, they shift to anterior and posterior positions, respectively. As the vagus nerves continue into the abdomen, they are named the anterior and posterior vagal trunks.

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Fig. 7.30   Esophageal arteries

Anterior view.

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Fig. 7.31   Esophageal veins

Anterior view.

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Lymphatics of the Mediastinum

image The superior phrenic lymph nodes drain lymph from the diaphragm, pericardium, lower esophagus, lung, and liver into the bronchomediastinal trunk. The inferior phrenic lymph nodes, found in the abdomen, collect lymph from the diaphragm and lower lobes of the lung and convey it to the lumbar trunk. Note: The pericardium may also drain superiorly to the brachiocephalic lymph nodes.

Fig. 7.32   Lymph nodes of the mediastinum and thoracic cavity

Left anterior oblique view.

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Fig. 7.33   Lymphatic drainage of the heart

A unique “crossed” drainage pattern exists in the heart: lymph from the left atrium and ventricle drains to the right venous junction, whereas lymph from the right atrium and ventricle drains to the left venous junction.

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image The paraesophageal nodes drain the esophagus. Lymphatic drainage of the cervical part of the esophagus is primarily cranial, to the deep cervical lymph nodes and then to the jugular trunk. The thoracic part of the esophagus drains to the bronchomediastinal trunks in two parts: the upper half drains cranially, and the lower half drains inferiorly via the superior phrenic lymph nodes. The bronchopulmonary and paratracheal nodes drain lymph from the lungs, bronchi, and trachea into the bronchomediastinal trunk (see p. 118).

Fig. 7.34   Mediastinal lymph nodes

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