As you may well know, the aorta is the largest artery in the body. It is responsible for transporting oxygenated blood from the left ventricle of the heart to the rest of the body via smaller arteries that branch off of the aorta. In this article, we will look at the anatomy of the aorta including its segments and location. We will also describe the arteries that branch off of each of the four different sections of the aorta, including the areas of the body to which they supply blood.
Segments of the Aorta
The aorta can be segmented in different ways. The first divides the aorta into thoracic and abdominal sections. The thoracic aorta is the segment that lies above the diaphragm within the thoracic cavity. The thoracic cavity also holds the heart and the lungs. The abdominal aorta therefore is the segment of the aorta that lies below the diaphragm; within the abdominal cavity which also houses the rest of the internal organs. While dividing the aorta this way will work fine as a learning strategy, it can mean having to learn and associate more arterial branches with each of these segments than if a second strategy is used.
The second strategy further divides the thoracic aorta into three parts (segments):
2) Arch, and
3) Descending Thoracic
Our discussion of the aorta then will be based on these three segments (ascending, arch, and descending thoracic), and the fourth segment, the abdominal aorta. Note: the descending aorta has both thoracic and abdominal portions.
The ascending aorta exits the left ventricle of the heart at the level of the lower border of the costal cartilage (the cartilage between the end of the rib and the sternum) of the third rib, just posterior to the left half of the sternum. The ascending aorta then extends superiorly, moving slightly forward, and to the right. It continues until it reaches the level of the costal cartilage of the second rib on the right.
From here, the aorta continues as the arch of the aorta. The arch extends upwards, as high as the midline of the manubrium of the sternum, passes over the right pulmonary artery as it moves posteriorly, all while moving towards the left side of the sternum. The arch then turns inferiorly to descend as far as thoracic vertebral level 4/5 (T4/5) and is now located to the left of the sternum. The arch is initially anterior to the trachea, but ends to the left of the trachea and the vertebral column.
The thoracic component of the descending aorta continues inferiorly until it reaches the lower edge of thoracic vertebra 12 (T12). In its pathway, the descending thoracic aorta moves medially, where at the level of T12, it is located directly anterior to the vertebral column. The aorta then passes posterior to the diaphragm within the aortic hiatus (hiatus = gap) to descend into the abdominal cavity.
The aorta descends through the abdomen and reaches its terminal branches (the right and left common iliac arteries) at the level of the fourth lumbar vertebra (L4). The terminal branches are the result of the aorta bifurcating, or splitting. At this point, the aorta is again situated slightly to the left of the vertebral column and the bifurcation can be visualized as lying about 2.5cm below the umbilicus (belly button).
Branches of the Ascending Aorta
There are only two arteries that branch off of the ascending aorta, they are the left and right coronary arteries. They can sometimes be used as trick questions on exams as students often forget that there are any arteries that arise from the ascending aorta at all! These are important branches however, as they supply oxygenated blood to the myocardium (the muscular layer of the heart wall). If these arteries become blocked, blood supply to the heart muscle can become compromised, and without an oxygenated blood supply, the heart muscle can begin to die as occurs in a myocardial infarctions, otherwise known as heart attacks.
Branches of the Arch of the Aorta
There are three arteries that branch off of the arch of the aorta. In the order in which they branch off of the arch, they are: 1) the brachiocephalic trunk/artery, 2) the left common carotid and 3) the left subclavian arteries.
- Brachiocephalic Trunk/Artery: This artery itself gives off two branches, the right subclavian and the right common carotid arteries. The right subclavian artery passes laterally, superior to the clavicle, and then descends into the arm by running underneath the clavicle (sub = below, clavian = clavicle) and supplies oxygenated blood to the right arm. The right common carotid artery runs superiorly up the right side of the neck, posteromedial to the internal jugular vein, and supplies oxygenated blood to the right side of the head (including the brain) and neck.
- Left Common Carotid Artery: Unlike the right common carotid artery, the left branches directly from the arch of aorta. Like the right side, the left common carotid runs superiorly up the left side of the neck and supplies the head and neck on the left side.
- Left Subclavian: This is the third branch off of the arch of the aorta. Parallel to the right side, the left subclavian initially passes laterally above the clavicle and then descends into the left arm by passing below the clavicle to supply oxygenated blood to the left arm.
Another structure of interest associated with the arch of the aorta is the ligamentum arteriosum. In fetal circulation, this ligament is actually a blood vessel (ductus arteriosus) that allows blood exiting the right ventricle, via the pulmonary trunk, to bypass the lungs and drain directly into the aorta. This vessel closes soon after birth, and becomes ligamentous. This ligament is also a helpful landmark for locating a branch of the 10th cranial nerve (the vagus nerve - CN10), the left recurrent laryngeal nerve, as it passes posterior and lateral to the ligamentum arteriosum to run underneath the arch of the aorta. This nerve can sometimes be compressed by enlarged lymph nodes, associated with the spread of lung cancer, resulting in paralysis of the vocal cords and a hoarseness of the voice.
Branches of the Descending Thoracic Aorta
Not often discussed are a number of arterial branches arising from the descending aorta within the thoracic cavity:
- Pericardial branches: A few vessels that supply the posterior aspect of the pericardial sac surrounding the heart.
- Bronchial branches: Typically, but not always, two left branches and one right branch that supply blood to components of the bronchial tree at and below the level of the primary bronchi.
- Esophageal branches: Four or five vessels that supply blood to the esophagus.
- Mediastinal branches: Several branches that supply blood to lymph nodes, nerves, vessels and areolar tissue located in the posterior mediastinum.
- Posterior Intercostal Arteries and Subcostal Artery: Include ten pairs of arteries that arise from the posterior aspect of the descending aorta and run between the ribs and supply the intercostal (inter = between, costal = ribs) and subcostal spaces below the 12th rib.
- Superior Phrenic Arteries: Arise from the lower part of the descending thoracic aorta to supply the posterior aspect of the superior surface of the diaphragm.
Branches of the Abdominal Aorta
There are a number of branches associated with the abdominal aorta, and they can be grouped in a number of ways for learning purposes. First they can be grouped as visceral (associated with organs), posterior, and terminal branches. Or they can be grouped strictly based on where they arise from off of the aorta (anterior, lateral, posterior). Finally, they can be learned in the order in which they branch off of the aorta from superior to inferior. This is the strategy we will use as it is most helpful when finding these structures on a cadaveric specimen:
- Inferior Phrenic Arteries: Paired branches (right and left) that arise from the lateral aspect of the aorta immediately below the aortic hiatus in the diaphragm to supply the inferior surface of the diaphragm.
- Celiac Trunk: A single branch (unpaired) that arises from the anterior surface of the aorta just below the aortic hiatus in the diaphragm. It will supply blood directly to the foregut (parts of the intestinal tract and accessory organs that lie below the diaphragm: from the abdominal esophagus to the first part of the duodenum of the small intestine – including the stomach, liver, pancreas, gallbladder, and spleen). It also supplies blood to the spleen, liver, and stomach via splenic, hepatic, and gastric arterial branches.
- Middle Suprarenal Arteries: Paired branches that arise laterally from the aorta inferior to the celiac trunk. As their name implies, they supply blood to the glands that lie on top of the kidneys, the suprarenal glands.
- Superior Mesenteric Artery: An unpaired branch arising from the anterior surface of the aorta inferior to the celiac trunk. It supplies blood to the midgut, or 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, the cecum, appendix, ascending colon and the first 2/3 of the transverse colon.
- Renal Arteries: Paired branches that arise from the lateral surface of the aorta immediately below the superior mesenteric artery. They supply blood to the left and right kidney.
- Testicular or Ovarian Arteries: These paired branches arise from the anterior aspect of the aorta just inferior to the renal arteries. As their name suggests, they supply blood to either the testes in the male or the ovaries in the female.
- Inferior Mesenteric Artery: This unpaired branch arising from the anterior surface of the aorta supplies the remaining segments of the intestinal tract or the hindgut. This includes the distal 1/3 of the transverse colon, descending colon, sigmoid colon, rectum, and the superior portion of the anal canal.
- Lumbar Arteries: Typically these are four pairs of arteries that arise from the posterior surface of the aorta. They will supply the posterior abdominal wall and the spinal cord.
- Median Sacral Artery: Arises posteriorly from the aorta just superior to the bifurcation. Supplies blood to the lower lumbar vertebrae and the sacrum.
- Common Iliac Arteries: The abdominal aorta ends as it bifurcates (typically at the level of the L4 vertebra) into its two terminal branches, the right and left common iliac arteries. The common iliac and its branches will supply the lower limb, the gluteal region, and the pelvic viscera.
Provided below, as a summary, is a list of the branches of the aorta, the segment with which they are associated, and the structures that are supplied by the branches.
- Coronary arteries (left and right): Myocardium
Arch of the Aorta
- Brachiocephalic trunk/artery: Right upper limb, head and neck
- Left common carotid artery: Left head and neck
- Left subclavian artery: Left upper limb
Descending Thoracic Aorta
- Pericardial branches: Posterior aspect of the pericardial sac
- Bronchial branches: Bronchial tree below the trachea
- Esophageal branches: Esophagus
- Mediastinal branches: Lymph nodes, nerves, vessels, areolar tissue of the posterior mediastinum
- Posterior intercostal and subcostal arteries: Intercostal/subcostal spaces
- Superior phrenic arteries: Superior surface of the diaphragm
- Inferior phrenic arteries: Inferior surface of the diaphragm
- Celiac trunk: Foregut (abdominal esophagus to proximal duodenum) including liver, pancreas, gallbladder, and spleen
- Middle suprarenal arteries: Suprarenal glands
- Superior mesenteric artery: Midgut (proximal duodenum to distal 1/3 of transverse colon)
- Renal arteries: Kidneys
- Testicular or ovarian arteries: Testes or ovaries
- Inferior mesenteric artery: Hindgut (distal 1/3 of transverse colon to superior portion of the anal canal)
- Lumbar arteries: Posterior abdominal wall and spinal cord
- Median sacral artery: Lower lumbar vertebrae and sacrum
- Common iliac arteries (right and left): Lower limbs, pelvic organs, gluteal region