A Projection of the heart and vessels onto the chest wall
Anterior view. The two great arterial vessels in the thorax are the aorta and the pulmonary trunk. Because the pulmonary arteries run a very short distance before entering the lungs, they are discussed under the heading of the pulmonary vessels (see p. 150f).
The ascending aorta is “in the shadow” of the sternum on the PA chest radiograph, while the aortic arch (“aortic knob”) forms the superior left portion of the left heart border. The descending aorta is hidden by the heart itself.
B Parts of the aorta and their relationship to the trachea and esophagus
Left lateral view. The aorta consists of three main parts:
• Ascending aorta: arises from the left ventricle, is dilated near the heart to form the aortic bulb (not visible here).
• Aortic arch: the arched portion of the aorta between the ascending and descending parts, runs posteriorly and to the left. A constriction may persist as an embryonic remnant in this part of the aorta (the aortic isthmus, see p. 198).
• Descending aorta: consists of the thoracic and abdominal portions of the aorta (see D).
C Functional groups of arteries that supply the thoracic organs
These are mainly vessels that supply the organs and internal structures of the thorax. The intrathoracic branches of the aorta can be divided into four main functional groups:
Arteries to the head and neck or to the upper limb:
• Brachiocephalic trunk with
– Right common carotid artery
– Right subclavian artery
• Thyroid ima artery (present in only 10% of the population)
• Left common carotid artery
• Left subclavian artery
Direct aortic branches that supply intrathoracic structures:
• Visceral branches to thoracic organs (heart, trachea, bronchi, and esophagus):
– Right and left coronary arteries
– Tracheal branches
– Pericardial branches
– Bronchial branches
– Esophageal branches
• Parietal branches to the internal (mainly posterolateral) chest wall and diaphragm:
– Posterior intercostal arteries
– Right and left superior phrenic arteries
Indirect paired branches (not arising directly from the aorta) that are distributed primarily to the head and neck but give off branches, usually small, that enter the chest and supply intrathoracic organs:
• Inferior thyroid artery (from the thyrocervical trunk = branch of subclavian artery) with
– Esophageal branches
– Tracheal branches
Indirect paired branches which supply the chest wall (mostly anterior, some inferior), usually in the form of parietal branches, and may give off other branches to intrathoracic organs (visceral sub-branches):
• Internal thoracic artery (from the subclavian artery) with
– Thymic branches
– Mediastinal branches
– Anterior intercostal branches
– Pericardiacophrenic artery (with branches to the pericardium and diaphragm)
– Musculophrenic artery (with a branch to the diaphragm)
D Position of the aorta in the thorax
Anterior view. The pleura, internal fasciae, and most thoracic organs have been removed, and the diaphragm has been windowed to display more of the thoracic cavity. The branches of the aorta (see C and p. 211) supply blood to all the organs, delivering almost 5 liters of blood per minute throughout the body. The thoracic aorta is thick-walled, particularly in its ascending segment and arch, but these walls are also elastic. During the systolic wave of pressure as the left ventricle contracts, these segments of the aorta dilate rapidly and then recoil. This serves to absorb and dissipate the pressure wave to produce a steadier, more even flow of blood in the arteries farther away from the heart. Because the aortic arch runs posteriorly and to the left, the relationship of the aorta to the trachea and esophagus changes as the vessel passes inferiorly through the chest (see also B and p. 170). The most anterior part of the aorta is the ascending aorta. The aortic arch then passes to the left side of the trachea, arching over the left main bronchus. It passes initially to the left of the esophagus but then descends posterior to the esophagus and anterior to the vertebral column. Because of this relationship, an abnormal outpouching of the aortic wall (aneurysm) may narrow the esophagus and cause swallowing difficulties (dysphagia). The thoracic aorta pierces the diaphragm at the aortic hiatus (junction of the T11/T12 vertebrae), becoming the abdominal aorta.
Note: In rare cases the aortic arch is constricted behind the ligamentum arteriosum (see B). This constriction is normal in the embryonic circulation, but its persistence after birth may produce the clinical manifestations of a coarctation of the aorta. This includes hypertension in the head, neck, and upper limbs, insufficient blood flow in the lower extremities, and left ventricular hypertrophy (due to chronic excessive workload and pressure) (see p.198 f).
E Aortic Windkessel function
a During systole, part of the ventricular stroke volume is stored in the elastic wall of the aorta (blue arrows pointing outward) and discharged again during diastole (b) (blue arrows pointing inward).
A Projection of the venae cavae onto the skeleton
Anterior view. The superior vena cava lies to the right of the midline and appears at the right sternal border on radiographs. Formed by the confluence of the two brachiocephalic veins, the superior vena cava enters the right atrium of the heart from above, forming its border in the PA chest radiograph (see p. 110).
The inferior vena cava runs a very short distance within the thorax (approximately 1 cm, not shown here). Immediately after piercing the diaphragm (at the caval opening), it passes through the pericardium and ends by opening into the right atrium of the heart from below. It has no tributaries within the chest (the pulmonary veins are described on p. 150f).
B The azygos system
Anterior view. The venous drainage of the thorax is handled mainly by the long azygos system, which runs vertically through the chest. The azygos vein runs to the right of the vertebral column, the hemiazygos vein to the left. The hemiazygos vein empties into the azygos vein, which in turn empties into the superior vena cava. An accessory hemiazygos vein is frequently present in the upper left thorax; it may open independently into the azygos vein or by way of the hemiazygos vein. The azygos system receives tributaries from the mediastinum and from portions of the chest wall, predominantly in the central and lower thorax.
Note: The azygos vein empties into the superior vena cava, while the ascending lumbar veins on both sides open into the inferior vena cava via the lumbar veins and the common iliac veins. In this way the azygos system creates a shunt between the superior and inferior venae cavae, called the “cavocaval anastomosis.” If drainage from the inferior vena cava is obstructed, venous blood can still reach the superior vena cava and enter the right heart by passing through the azygos system (see D and p. 218).