Aorta

The aorta is the principal and largest artery of the human body. Originating at the left ventricle of the heart, it delivers oxygenated blood to the systemic circulation and supplies every region of the body through a vast network of branching arteries. Structurally complex and functionally vital, the aorta traverses both the thoracic and abdominal cavities before bifurcating into the common iliac arteries.

Structure and Divisions

The aorta begins at the aortic valve and ascends briefly within the pericardial sac as the ascending aorta. It then curves posteriorly and to the left, forming the aortic arch, before descending along the vertebral column as the descending aorta. This descending section is further divided into the thoracic aorta, located above the diaphragm, and the abdominal aorta, situated below it. The artery terminates at the aortic bifurcation, dividing into the left and right common iliac arteries, with the median sacral artery arising as a smaller midline branch.
Another method of classification considers the aorta in relation to blood flow: ascending aorta, arch, and descending sections. Each part gives rise to distinct arterial branches critical for supplying the heart, thoracic organs and abdominal viscera. Throughout its course the aorta lies close to structures such as the pulmonary trunk, the trachea, the oesophagus and major autonomic nerve plexuses.

Ascending Aorta

The ascending aorta begins at the opening of the aortic valve. Initially positioned posterior to the pulmonary trunk, it twists to lie anterior and to the right as the two vessels spiral upwards. The root of the ascending aorta features three aortic sinuses, known as the sinuses of Valsalva. The left and right sinuses give rise to the left and right coronary arteries respectively, while the posterior or non-coronary sinus does not form an arterial branch. These coronary vessels supply the myocardium and represent the first branches of the systemic circulation.

Aortic Arch

The arch passes over the left pulmonary artery and the bifurcation of the pulmonary trunk, linked to the latter by the ligamentum arteriosum, a remnant of the fetal ductus arteriosus. It crosses anterior to the left main bronchus and is closely associated with autonomic fibres forming the cardiac or aortic plexus. The left recurrent laryngeal nerve, a branch of the vagus nerve, hooks beneath the arch adjacent to the ligamentum arteriosum before ascending back to the larynx.
Three major branches arise from the arch, arranged from proximal to distal:

• Brachiocephalic trunk, which divides into the right common carotid and right subclavian arteries
• Left common carotid artery
• Left subclavian artery

These branches supply the head, neck and upper limbs. The arch transitions into the descending aorta around the level of the fourth to fifth thoracic vertebrae.

Thoracic Aorta

Descending through the thoracic cavity, the aorta gives rise to several important branches, including:

• Posterior intercostal arteries (third to eleventh) and the subcostal artery for the thoracic wall
• Bronchial arteries, typically superior and inferior, supplying the bronchi
• Oesophageal branches to the thoracic oesophagus
• Pericardial and mediastinal branches
• Superior phrenic arteries supplying the diaphragm

These vessels contribute extensively to the vascular network of the thoracic organs and body wall.

Abdominal Aorta

The abdominal aorta begins after the artery passes through the aortic hiatus of the diaphragm. Its branches include:
Parietal branches

• Inferior phrenic arteries
• Lumbar arteries
• Median sacral artery

Visceral branches

• Coeliac trunk, supplying the foregut organs
• Middle suprarenal arteries
• Renal arteries to the kidneys
• Gonadal arteries (testicular in males, ovarian in females)
• Superior mesenteric artery, supplying the midgut
• Inferior mesenteric artery, supplying the hindgut

The abdominal aorta ends at the level of the fourth lumbar vertebra by dividing into the common iliac arteries.

Development

During embryonic development, the ascending aorta arises from the outflow tract of the primitive heart tube. Through a complex process of remodelling, the symmetrical aortic arches develop into the mature asymmetric aortic pattern. The third pair contributes to the common carotid arteries; the right fourth arch forms part of the right subclavian artery; and the left fourth arch becomes the central portion of the aortic arch.
The smooth muscle cells of the ascending aorta and those forming the aorticopulmonary septum are derived from the neural crest, in contrast to the mesoderm-derived smooth muscle of the abdominal aorta and coronary arteries. Developmental errors may result in anomalies such as persistent truncus arteriosus.

Microanatomy

The aorta is a classic example of an elastic artery. Its wall comprises three layers:

• Tunica intima, containing endothelial cells and connective tissue
• Tunica media, the thickest layer, featuring concentric musculoelastic lamellae composed of smooth muscle, elastic fibres, collagen (notably type III), proteoglycans and glycosaminoglycans
• Tunica externa, containing connective tissue, nerves and the vasa vasorum, the small vessels supplying the outer layers of the aortic wall

The elastic matrix provides resilience and enables the aorta to dampen the pulsatile output of the heart, ensuring steady blood flow downstream. Baroreceptors and chemoreceptors in the aortic arch monitor blood pressure and blood chemistry, transmitting information to the medulla oblongata to maintain homeostasis.

Variations

The aorta and its branches display numerous anatomical variations. It may be found on the right side in conditions such as dextrocardia or situs inversus. Branching patterns vary: the left vertebral artery may arise directly from the aorta rather than the left subclavian artery, and abnormal persistence of the ductus arteriosus may result in a patent ductus arteriosus, creating an abnormal connection between the pulmonary artery and the descending aorta.
Branching patterns from the aorta include the following major arteries:

• Right and left coronary arteries
• Brachiocephalic, common carotid and subclavian arteries
• Bronchial, oesophageal and posterior intercostal arteries
• Inferior phrenic, lumbar and median sacral arteries
• Coeliac trunk, superior mesenteric, renal, gonadal and inferior mesenteric arteries
• Terminal common iliac arteries