Anatomy, Thorax, Superior Vena Cava

The superior vena cava is a large, significant vein responsible for returning deoxygenated blood collected from the body back into the heart. It is present within the superior and middle mediastinum. The superior vena cava handles the venous return of blood from structures located superior to the diaphragm. The inferior vena cava handles venous return from the portion of the body inferior to the diaphragm. The superior vena cava is a commonly used site for central venous access.[1]

The right and left brachiocephalic veins, also known as the innominate veins, join to form the superior vena cava posterior to the inferior border of the first right costal cartilage. These veins carry blood from the chest wall, esophagus, lungs, neck, and pericardium. The superior vena cava descends downwards vertically, running posterior to the second and third intercostal spaces. The azygos vein drains into the superior vena cava via the azygos arch when it is behind the first intercostal space, just prior to the point at which it penetrates the fibrous pericardium that is located opposite the right second costal cartilage. It drains into the right atrium at the level of the third costal cartilage. The lower portion of the superior vena cava is covered by this fibrous pericardium. The superior vena cava terminates in the superior and posterior portion of the sinus venarum of the right atrium. There are no valves present at the point at which the superior vena cava drains into the right atrium. The superior vena cava is approximately 7 centimeters in length and 2 cm wide.[2]

The superior vena cava derives from the proximal portion of the right anterior cardinal vein and the right common cardinal vein at a point that is caudal to the transverse anastomosis in the embryo. During week eight of development, a large anastomosis that formed via the thymic and thyroid veins shunts blood from the left anterior cardinal vein towards the right. Superior to this anastomosis, the anterior cardinal veins become the internal jugular veins. The cardinal veins are responsible for venous return; they represent the primary drainage system during embryonic development. The anterior cardinal veins drain the cephalic portion of the embryo while the posterior cardinal veins drain the remaining embryonic blood. The anterior and posterior cardinal veins merge before entering the sinus venosus. At this point, they become the short common cardinal veins. The right brachiocephalic vein forms from the right anterior cardinal vein at a point that is cranial to the transverse anastomosis. The left brachiocephalic vein forms from a portion of the left anterior cardinal vein and transverse intercardinal anastomosis. During embryonic development, several malformations may occur including left superior vena cava, double superior vena cava, as well as improper pulmonary venous drainage into the super vena cava. 

The right and left brachiocephalic, or innominate, veins converge to form the superior vena cava at the level of the right first costal cartilage. These veins form at the venous angle from a merging of the subclavian and jugular veins posterior to the sternoclavicular joints. The brachiocephalic veins carry deoxygenated blood that is returning to the heart from the pericardium, lungs, esophagus, chest wall, and neck. The left brachiocephalic vein is more horizontal in its orientation and is longer than the more vertical and shorter right brachiocephalic vein. The right brachiocephalic vein is approximately 2 to 3 cm long and sits anterior to the innominate artery. The left brachiocephalic vein is approximately 6 cm long and lies anterior to the left subclavian vein and common carotid arteries, posterior to the manubrium, and superior to the aortic arch. The left superior intercostal vein joins with the left brachiocephalic vein; the azygos vein joins with the right brachiocephalic vein.  The vertebral, internal thoracic, and inferior thyroid veins are all tributaries of the brachiocephalic veins.[2]

The azygos vein is another vein that drains into the superior vena cava. This vein is unpaired. It runs along the right aspect of the thoracic vertebral column and enters into the thorax at the aortic hiatus of the diaphragm. The azygos vein forms from the joining of the right subcostal and ascending lumbar veins. The hemiazygos veins, bronchial veins, pericardial veins, esophageal veins, and posterior right intercostal veins all drain into the azygos vein.

Sympathetic axons originating from the right stellate ganglion descend along the superior vena cava. Most of these axons travel to the posterior surface of the heart. However, some extend onto the anterior side of the heart. These axons provide sympathetic innervation for the sinoatrial node. Cardiac parasympathetic nerves also run along the length of the superior vena cava. Efferent vagal nerve fibers travel through a pad of fat located in between the proximal aorta and medial superior vena cava.[3][4][5]