The aorta is the first segment of the systemic arterial circulation, originating directly from the left ventricle of the heart. It is the largest artery in the body consisting of three parts that each has its special characteristics, most notably in their direction and orientation. At the beginning of its course, the aorta runs upwards as the ascending aorta then shortly after arches laterally to the left, forming the arch of aorta. After that, the aorta continues downwards as the descending aorta until it divides into its terminal branches.
The aorta is the main artery in the body whose only and important function is to carry oxygenated blood from the heart to the rest of the body. It is important to note that all the arteries of the human body (except the pulmonary arteries), no matter where they are, receive blood from the aorta. This becomes possible by the numerous branches that aorta gives off along its course.
This article will discuss the anatomy and function of the aorta.
Left and right coronary artery
Arch of the aorta:
Left common carotid artery
Left subclavian artery
Descending thoracic aorta:
Visceral branches: pericardial, bronchial, esophageal, mediastinal arteries
Parietal branches: intercostal, subcostal arteries and superior phrenic arteries
Descending abdominal aorta:
Anterior group: celiac trunk, superior mesenteric, inferior mesenteric arteries
Lateral group: suprarenal, renal, gonadal (testicular in males, ovarian in females) arteries
Dorsal group: inferior phrenic arteries, lumbar arteries, median sacral artery
Terminal branches: left and right common iliac artery
- Definition and location
- Ascending aorta
- Aortic arch
- Descending aorta
- Aortic aneurysm
Definition and location
Being the first segment of the systemic circulation, the aorta originates directly from the heart. It begins from the aortic orifice found at the base of the left ventricle. It is separated from the ventricle by the aortic valve, that consists of three semilunar cusps; the right, left and posterior cusp. The aorta is approximately 30 cm long, spanning almost the entire length of the trunk. It is the largest artery in the body, with its largest diameter being approximately 4 cm at the aortic root decreasing with distance; being about 3.5 cm in the ascending aorta, and about 2.5 cm in the abdominal aorta. The main function of the aorta is to transmit oxygenated blood from the heart to the rest of the body.
There are a number of classifications of the aorta. Most common one, and the one we will use in this article, is the classification of the aorta into three parts; the ascending aorta, aortic arch and the descending aorta. The descending aorta is the longest part, further subdivided into thoracic and abdominal aorta, with the diaphragm being the anatomical division between the two.
Some authors use the term “descending aorta” to describe the thoracic aorta, and hence use the classification which recognizes four parts of the aorta; the ascending aorta, aortic arch, descending aorta, and abdominal aorta.
In addition, the ascending aorta, aortic arch and the thoracic part of the descending aorta are sometimes together referred to as the thoracic aorta, due to them all being confined to the thoracic cavity. This classification describes the aorta as having two parts; the thoracic and abdominal aorta.
The ascending aorta is the first part of the aorta that begins at the aortic orifice on the base of the left ventricle, roughly at the level of the lower border of the third left costal cartilage. From this point, it ascends obliquely superiorly and slightly laterally to the right, and ends at the sternal angle, just before the branching of the brachiocephalic trunk.
The sternal angle is an angle formed between the sternum and the second costal cartilage. The ascending aorta is the widest portion of the aorta with a diameter of approximately 3 cm and a length of 5 cm.
The ascending aorta gives off two arteries, which, albeit few, are very important. These are the left and right coronary arteries that provide arterial supply to the heart. These arteries spring out of the origin point of the ascending aorta, a segment referred to as the aortic root. The aortic root contains three aortic sinuses, which are dilatations of the aortic wall found just above the cusps of the aortic valve. Viewed from above, these sinuses resemble three well defined rounded pockets that meet at the center of the aortic lumen. The aortic sinuses, also known as Valsalva sinuses, are named according to their position as the anterior, left posterior, and right posterior aortic sinuses.
- Anterior aortic sinus: occupies the space between the wall of the aorta and the right cusp of the aortic valve. This sinus is also called the right coronary sinus as it gives rise to the right coronary artery.
- Left posterior aortic sinus: occupies the space between the wall of the aorta and the left cusp of the aortic valve. This sinus is also called the left coronary sinus as it gives rise to the left coronary artery.
- Right posterior aortic sinus: occupies the space between the wall of the aorta and the posterior cusp of the aortic valve. This sinus is also called the non-coronary sinus since no vessels arise from it.
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Along with the heart, the ascending aorta is situated in the inferior mediastinum. The ascending aorta is completely enclosed by pericardium together with the pulmonary trunk. These two vessels initially run in parallel, with the infundibulum of the pulmonary trunk lying anterior to the ascending aorta. Soon after, the pulmonary trunk and the ascending aorta twist around each other so that the ascending aorta ends up being anterior.
Posterior relations of the ascending aorta include the left atrium and pulmonary trunk on the left, and right pulmonary artery and principal bronchus on the right. Laterally to the right is the superior vena cava, and the right atrium, and laterally to the left is the left atrium and the pulmonary trunk. Superiorly, the ascending aorta communicates with the remaining tissue of the thymus gland, the sternum, the right pleura, and the anterior margin of the right lung.
The aortic arch is the second part of the aorta that begins at the branching point of the brachiocephalic trunk, posterior to the right second sternocostal joint. In its course, the aortic arch curves upwards and backwards to the left, anterior to the bifurcation of the trachea. It continues so until it arches downwards, running posterior to the left root of the lung.
Upon reaching the T4 vertebra, the aortic arch terminates after it gives off the left subclavian artery, roughly at the level of the sternal angle. As the aortic arch transitions into the thoracic part of the descending aorta, it forms a small stricture called the aortic isthmus, which is followed by a dilatation.
The convexity of the aortic arch gives off three branches; the brachiocephalic trunk, the left common carotid artery and the left subclavian artery.
- Brachiocephalic trunk: also called the brachiocephalic artery, is the first and largest artery that branches off the aortic arch. This artery courses upwards and slightly to the left until it reaches the level of the right sternoclavicular joint, where it divides into its two branches; the right subclavian and the right common carotid arteries. The right subclavian artery courses laterally, superior to the clavicle, and then descends underneath the clavicle to supply the right arm. The right common carotid artery courses upwards to supply the right side of the head and neck, including the brain.
- Left common carotid artery: unlike its right counterpart that arises from the brachiocephalic trunk, the left common carotid artery arises directly from the aortic arch. It branches off the aortic arch roughly at the level of the left sternoclavicular joint, after which it ascends lateral to the trachea and esophagus within the deep cervical fascia. The left and right common carotid arteries follow the same course on the opposite sides of the neck before bifurcating into internal and external carotid arteries once they reach the level of the laryngeal thyroid cartilage. The left common carotid artery goes on to supply the left side of the head and neck.
- Left subclavian artery: this artery arises directly from the aortic arch, unlike the right subclavian artery that arises from the brachiocephalic trunk. Arising at the level of the sternal angle, the left subclavian artery is the last artery to be given off by the aortic arch, marking the termination of the aortic arch and the beginning of the descending aorta.
Over its entire course, the aortic arch lies in the superior mediastinum. The aortic arch has several important relations with the organs and neurovascular structures of the mediastinum.
Anteriorly and to the left, it communicates with the left lung and pleura. The left phrenic nerve, trunk of the vagus nerve, left cardiac nerves (superior cervical cardiac branch of the sympathetic trunk and inferior cardiac branch of left vagus nerve) and the left superior intercostal vein all cross the aortic arch anteriorly. Upon crossing the aortic arch, the vagus nerve gives off the recurrent laryngeal branch which wraps underneath the arch and emerges on the other side, ascending back towards the neck.
Posterior and to the right, the aortic arch is related to the trachea, esophagus, deep cardiac plexus, the left recurrent laryngeal nerve, thoracic duct and vertebral column. Superiorly, the aortic arch is related to the branches of the brachiocephalic trunk, as well as the left common carotid artery, left subclavian artery, left brachiocephalic vein and the thymus. Inferiorly, the aortic arch is related to the bifurcation of the pulmonary trunk, left principal bronchus, ligamentum arteriosum, superficial cardiac plexus and left recurrent laryngeal nerve.
The descending aorta is the largest part of the aorta. It arises as a continuation of the aortic arch after the branching of the left subclavian artery. As previously mentioned, the descending aorta is divided into the thoracic aorta and the abdominal aorta, which are anatomically separated by the diaphragm.
The thoracic aorta begins at the level of the T4 vertebra and courses downwards through the posterior mediastinum. Initially, it is found left to the vertebral column, but as it descends it inclines towards the midline and ends up being anterior to the lower border of the body of T12 vertebra. At this level, it transitions into the abdominal aorta and passes through the aortic orifice of the diaphragm into the abdominal cavity.
The branches of the thoracic aorta can be divided into visceral branches, intended largely for the organs of the mediastinum, and parietal branches, intended for the structures of the thoracic wall. The visceral branches are the pericardial, bronchial, esophageal and mediastinal branches, while the parietal branches are intercostal, subcostal and superior phrenic arteries.
- Pericardial branches: consist of a few vessels that supply the posterior aspect of the pericardium.
- Bronchial arteries: they are usually one right and two left bronchial arteries. The artery on the right side supplies the right bronchus, bronchopulmonary lymph nodes, pericardium and esophagus. The two bronchial arteries on the left side supply blood to the components of the bronchial tree below the level of primary bronchi.
- Esophageal branches: consist of four or five vessels that supply blood to the esophagus.
- Mediastinal branches: consist of a few branches that supply blood to lymph nodes, nerves, vessels and areolar tissue located in the posterior mediastinum.
- Posterior intercostal arteries: consist of 9 pairs of arteries that run between the ribs and supply the intercostal spaces.
- Subcostal arteries: consists of a pair of arteries that run inferior to the costal margin on each side (inferior to the twelfth ribs) and supply the subcostal space.
- Superior phrenic arteries: consist of a pair of arteries that supply the posterior aspect of the superior surface of the diaphragm, anastomosing with the musculophrenic and pericardiacophrenic arteries.
The thoracic aorta is confined to the posterior mediastinum. Anteriorly, it is related to the left pulmonary hilum, pericardium that encloses the left atrium, the lower part of the esophagus and the diaphragm. Posteriorly, the thoracic aorta is in proximity with the vertebral column and hemiazygos and accessory hemiazygos veins. Laterally to the right is the upper part of the esophagus, the azygos vein, thoracic duct and the right lung and pleura. Laterally to the left is the left lung and pleura.
To clear out any confusion, the reason why the thoracic aorta has several different relations with the esophagus is due to the course of the esophagus through the mediastinum. The esophagus is initially laterally to the right of the thoracic aorta, then as it descends it crosses the thoracic aorta anteriorly. Finally, as it approaches the diaphragm ends up being anterolaterally to the thoracic aorta.
The abdominal aorta emerges from the aortic orifice of the diaphragm at the level of T12 vertebra. It begins its course anterior and to the left of the vertebral column and continues downwards for about 10 cm.
Upon reaching the lower part of the body of L4 vertebra, it bifurcates into its terminal branches to the left of the median plane; the left and right common iliac arteries. On the skin, this bifurcation is projected 2.5 cm below and to the left of the umbilicus.
Similarly to the branches of the thoracic aorta, the branches of the abdominal aorta can be grouped as visceral and parietal. However, a more commonly used classification is based on where the branches arise off of the aorta and are divided into anterior, lateral and dorsal groups.
Superior mesenteric artery
Inferior mesenteric artery
Inferior phrenic artery
Median sacral arteries
Left common iliac artery
Right common iliac artery
The anterior group of branches involves the celiac trunk, superior and inferior mesenteric artery;
- Celiac trunk: an unpaired short vessel that arises on the anterior aspect of the aorta just inferior to the aortic orifice of the diaphragm at the level of the lower border of T12 vertebra. Shortly after its origin, the trunk gives off its three branches; left gastric artery, common hepatic artery and splenic artery. These arteries supply the foregut, which is the part of the intestinal tract that lies below the diaphragm up to the proximal duodenum. The organs of the foregut that are supplied but the celiac trunk include the abdominal part of the esophagus, stomach, proximal duodenum, liver, gallbladder, pancreas, and spleen.
- Superior mesenteric artery: an unpaired artery that arises about 1 cm below the celiac trunk, at the level of L1 vertebra. It supplies blood to the midgut, which refers to the parts of the intestinal tract lying between the proximal segment of the duodenum and the distal 1/3 of the transverse colon of the large intestine. It, therefore, supplies most of the small intestine, cecum and appendix, ascending colon and the first 2/3 of the transverse colon.
- Inferior mesenteric artery: an unpaired artery that arises about 4 cm superior to the bifurcation of the abdominal aorta, at the level of L3 vertebra. It supplies blood to the hindgut, which refers to the remaining segments of the intestinal tract that include the distal 1/3 of the transverse colon, descending colon, sigmoid colon, rectum, and the superior portion of the anal canal.
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The lateral group of branches of the abdominal aorta involves the suprarenal, renal and gonadal arteries:
- Middle suprarenal artery: a paired artery that arises on each lateral side of the abdominal aorta near the origin of the superior mesenteric artery. Along with the superior and inferior suprarenal arteries, these arteries supply blood to the suprarenal glands.
- Renal artery: a large paired artery that branches off the lateral sides of the aorta at a right angle immediately below the superior mesenteric artery. It supplies blood to the left and right kidney and gives off the inferior suprarenal artery to supply the suprarenal glands.
- Gonadal artery: a paired artery that arises from the anterior aspect of the aorta just inferior to the renal arteries. In males, this artery is called the testicular artery and supplies the testes, while the female counterpart is called the ovarian artery, which supplies the ovaries.
The dorsal group of branches includes the inferior phrenic, lumbar, and median sacral arteries:
- Inferior phrenic artery: a paired artery that arises from the posterolateral aspect of the aorta immediately below the aortic hiatus of the diaphragm. This artery supplies the inferior surface of the diaphragm and gives off the superior suprarenal artery to supply the suprarenal glands.
- Lumbar arteries: typically four pairs of arteries that arise from the posterior surface of the aorta. They supply the posterior abdominal wall and the spinal cord.
- Median sacral artery: arises from the posterior aspect of the abdominal aorta just superior to the bifurcation. This artery supplies blood to the lower lumbar vertebrae and the sacrum.
The terminal branches of the abdominal aorta, the left and right common iliac arteries, arise from the bifurcation in front of the body of L4 vertebra about 1.25 cm to the left of the median plane. The common iliac arteries supply the lower limb, the gluteal region, and the pelvic viscera.
The abdominal aorta courses through the retroperitoneal space of the abdominal cavity. Posteriorly, it is related to the vertebral bodies of the upper four lumbar vertebrae and the overlying anterior longitudinal ligament. Anteriorly, the abdominal aorta is related to the organs of the abdominal cavity. From above downward, it relates to the pancreas, the descending part of the duodenum, the root of the mesentery, and the small intestine. Several neurovascular structures are also related to the abdominal aorta; anteriorly, it is related to the splenic and renal veins; laterally and to the right, it relates to the inferior vena cava, and laterally to the left with the sympathetic trunk.
An aortic aneurysm is a localized enlargement of the aortic wall. It is most commonly developed in the abdominal aorta, followed by the ascending aorta and aortic arch. An aortic aneurysm is diagnosed when the increase in aortic wall diameter is 1.5 times more than its normal size. Since the normal diameter of the ascending aorta amounts to about 3.5 cm, and 2.5 cm for the abdominal aorta, the diagnostic criteria for an aneurysm include a diameter of >5cm for the ascending aorta, and >4cm for the abdominal aorta.
The abdominal aorta is the most commonly affected segment of the aorta due to the fact that it contains less elastin in its wall compared to the ascending aorta, as well as the fact that it lacks vasa vasorum, making it susceptible to degenerative changes. The most common cause of an abdominal aorta is atherosclerosis, while aneurysms in the ascending aorta and aortic arch commonly occur secondary to connective tissue disorders such as Marfan’s syndrome and Ehlers–Danlos syndrome.
An aortic aneurysm is often asymptomatic, only sometimes causing signs and symptoms that are mostly the result of the aneurysm affecting nearby structures. An aneurysm in the ascending aorta or aortic arch may cause shortness of breath and cough due to compression of the lungs; or voice hoarseness due to stretching of the recurrent laryngeal nerve. Likewise, an aneurysm of the abdominal aorta may compress the spinal nerve roots and cause numbness in the lower extremities.
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