The great vessels of the heart function to carry blood to and from the heart as it pumps, located largely within the middle mediastinum.
In this article we will consider the structure and anatomical relationships of the aorta, pulmonary arteries and veins, and the superior and inferior vena cavae.
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Aorta
The aorta is the largest artery in the body. It carries oxygenated blood (pumped by the left side of the heart) to the rest of the body.
The aorta arises from the aortic orifice at the base of the left ventricle, with inflow via the aortic valve. Its first segment is known as the ascending aorta, which lies within the pericardium (covered by the visceral layer). From it branch the coronary arteries. The second continuous segment is the arch of the aorta, from which branch the major arteries to the head, neck and upper limbs. These are:
- Brachiocephalic trunk
- Left common carotid artery
- Left subclavian artery
After the arch of the aorta, the aorta then becomes the descending aorta which continues down through the diaphragm into the abdomen.
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Disorders of the Aorta
Aortic Dissection
Aortic dissection refers to a tear in the inner wall of the aorta. The tear creates two channels for blood flow; one is the normal lumen of the aorta, another is into the wall, where the blood becomes stationary.
Blood entering the wall can constrict the aortic lumen, reducing blood flow to the rest of the body. It can also cause further weakness and dilation of the wall, potentially leading to an aortic aneurysm.
Fig 2
Aortic dissection, where blood enters the wall of the aorta.
Aortic Aneurysm
An aneurysm is a dilation (expansion) of an artery, which is greater than 50% of the normal diameter. An aortic aneurysm is due to an underlying weakness of the walls (such as Marfan’s syndrome), or a pathological process (such as aortic dissection).
The main concern with an aortic aneurysm is rupture of the aorta, which if not treated, will lead to death.
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Pulmonary Arteries
The pulmonary arteries receive deoxygenated blood from the right ventricle and deliver it to the lungs for gas exchange to take place.
The arteries begin as the pulmonary trunk, a thick and short vessel, which is separated from the right ventricle by the pulmonary valve. The trunk is located anteriorly and medially to the right atrium, sharing a common layer of pericardium with the ascending aorta. It continues upwards, overlapping the root of the aorta and passing posteriorly.
At around the level of T5-T6, the pulmonary trunk splits into the right and left pulmonary arteries. The left pulmonary artery supplies blood to the left lung, bifurcating into two branches to supply each lobe of the lung. The right pulmonary artery is the thicker and longer artery of the two, supplying blood to the right lung. It also further divides into two branches.
Pulmonary Veins
The pulmonary veins receive oxygenated blood from the lungs, delivering it to the left side of the heart to be pumped back around the body.
There are four pulmonary veins, with one superior and one inferior for each of the lungs. They enter the pericardium to drain into the superior left atrium, on the posterior surface. The oblique pericardial sinus can be found within the pericardium, between the left and right veins.
The superior pulmonary veins return blood from the upper lobes of the lung, with the inferior veins returning blood from the lower lobes. The inferior left pulmonary vein is found at the hilum of the lung, while the right inferior pulmonary vein runs posteriorly to the superior vena cava and the right atrium.
Superior Vena Cava
The superior vena cava receives deoxygenated blood from the upper body (superior to the diaphragm, excluding the lungs and heart), delivering it to the right atrium.
It is formed by merging of the brachiocephalic veins, travelling inferiorly through the thoracic region until draining into the superior portion of the right atrium at the level of the 3rd rib.
As the superior vena cava makes its descent it is located in the right side of the superior mediastinum, before entering the middle mediastinum to lie beside the ascending aorta.
Inferior Vena Cava
The inferior vena cava receives deoxygenated blood from the lower body (all structures inferior to the diaphragm), delivering it back to the heart.
It is initially formed in the pelvis by the common iliac veins joining together. It travels through the abdomen, collecting blood from the hepatic, lumbar, gonadal, renal and phrenic veins. The inferior vena cava then passes through the diaphragm, entering the pericardium at the level of T8. It drains into the inferior portion of the right atrium.