Ascending Aorta
The first segment of the vessel is the ascending aorta (pars ascendens aortae). It begins at the aortic orifice (ostium aortae) in the left ventricle. Topographically, this segment is located posterior to the left half of the sternum, starting at the level of the third intercostal space.
The vessel courses superiorly, slightly to the right and anteriorly, reaching the right second costal cartilage. At this point, the ascending aorta smoothly transitions into the aortic arch.
The very beginning of the ascending segment is visibly dilated and is termed the aortic bulb (bulbus aortae). Within the bulb, the wall forms three characteristic pocket-like outpouchings known as the aortic sinuses (sinus aortae). Their positions strictly correspond to the attachments of the three cusps of the aortic semilunar valve.
Depending on their position, three sinuses are distinguished:
- Right aortic sinus (sinus aortae dexter) — gives rise to the right coronary artery.
- Left aortic sinus (sinus aortae sinister) — gives rise to the left coronary artery.
- Posterior aortic sinus (sinus aortae posterior) — is non-coronary (gives off no arterial branches).
Aortic Arch and Its Branches
The next segment is the aortic arch (arcus aortae). It extends from the right second costal cartilage to the left side of the bodies of the 3rd and 4th thoracic vertebrae. The arch is convex superiorly and concave inferiorly. At the junction where the arch terminates and the descending part begins, a physiological narrowing occurs, known as the aortic isthmus (isthmus aortae).
From the superior (convex) aspect of the arch, three major arterial trunks arise from right to left:
- Brachiocephalic trunk (truncus brachiocephalicus). This is a large vessel, approximately 4 cm long, running superiorly and to the right. Upon reaching the level of the right sternoclavicular joint, it divides into the right common carotid and right subclavian arteries. Rarely, it may give rise to the lowest thyroid artery (a. thyroidea ima). Anatomical variations include its absence (branches arising directly from the arch), origin from the left side, or duplication (separate right and left trunks).
- Left common carotid artery (a. carotis communis sinistra). It branches directly from the aortic arch, which makes it slightly longer than its counterpart on the right side.
- Left subclavian artery (a. subclavia sinistra). It courses superiorly and to the left, supplying blood to the neck region and upper extremity.
Descending Aorta
The descending aorta (pars descendens aortae) is the direct continuation of the aortic arch. It is the longest segment of the aorta, extending from the level of the 3rd–4th thoracic vertebrae down to the 4th lumbar vertebra.
Topographically and functionally, this extensive segment is divided into two halves by the diaphragm:
- Thoracic aorta (pars thoracica aortae) — located within the thoracic cavity down to the aortic hiatus of the diaphragm.
- Abdominal aorta (pars abdominalis aortae) — begins after the vessel passes through the aortic hiatus of the diaphragm (at the level of the 12th thoracic vertebra, Th12).
The descending aorta terminates at the level of the 4th lumbar vertebra by bifurcating into the right and left common iliac arteries (aa. iliacae communes dextra et sinistra). The main trunk sharply narrows and continues as the small median sacral artery (a. sacralis mediana), which descends along the anterior surface of the sacrum.