Anatomy, Abdomen and Pelvis, Aorta

Article Author:
Hunter White
Article Editor:
Judith Borger
Updated:
2/2/2019 2:37:11 PM
PubMed Link:
Anatomy, Abdomen and Pelvis, Aorta

Introduction

The aorta is the first and largest artery in the body; it is responsible for transporting oxygenated blood that after being ejected from the left ventricle to the rest of the body for gas exchange. The aorta initiates at the left ventricle and consists of several different segments. These segments include the ascending aorta, aortic arch, and the descending aorta which divides into the thoracic and abdominal portions.

Structure and Function

The aorta initiates at the top of the left ventricle, beginning with the aortic valve. Blood is pumped via the left ventricle through the aortic valve and into the aorta upon contraction. The aortic valve, which is between the left ventricle and aorta, is a three-chambered valve that allows for a unidirectional flow of blood out of the heart during contraction. This valve prevents the backflow of blood into the heart. From here the aorta extends upward. This portion of the aorta is known as the ascending aorta and is about 5 cm in total length. The aortic sinuses are at the origin of the ascending aorta opposite the aortic valve. The coronary arteries that branch off of the ascending aorta provide the heart with oxygenated blood. These arteries arise near the start of the aorta, just above the semilunar valves.

The location of the ascending aorta is within the pericardium. More specifically it is enclosed within the serous layer of the pericardium which contains the pericardial fluid. The aorta then curves upward, backward, and towards the left, passing over the root of the left lung and in front of the trachea to form the aortic arch. The arch originates at the upper border of the second sternocostal articulation and ends at the level of the body of the fourth thoracic vertebra where it merges into the descending aorta. The brachiocephalic, left common carotid, and left subclavian arteries all branch off the aortic arch. The brachiocephalic artery is the largest of the aortic arch branches and will divide into the right common carotid and right subclavian arteries. The aortic arch curves downward merging into the descending aorta at the level of the lower border of the fourth thoracic vertebra. The descending aorta divides into two portions, the thoracic aorta, and the abdominal aorta. The thoracic aorta is the portion of the descending aorta contained within the posterior mediastinal cavity that is continuous with the aortic arch. At its origin, this portion of the aorta lies just to the left of the vertebral column and drifts medially as it continues to move downward through the chest until it crosses the diaphragm.

The thoracic aorta gives rise to multiple branches which divide into visceral and parietal categories. The visceral branches include the pericardial, bronchial, esophageal, and mediastinal branches. The pericardial branches are small vessels which travel to the posterior surface of the pericardium. The left bronchial arteries, typically two in number, arise from the thoracic aorta as well. These arteries supply the bronchial tubes, bronchial lymph glands, esophagus, and areolar tissue of the lungs. The esophageal arteries arise from the anterior portion of the aorta and travel downward towards the esophagus where they anastomose with several other arteries. The mediastinal branches of the thoracic aorta go on to supply the lymph glands and areolar tissue located within the posterior mediastinum. The parietal branches include the intercostal, subcostal, and superior phrenic branches. There are nine pairs of intercostal arteries in total which arise from the posterior portion of the aorta. These arteries will divide into an anterior and posterior ramus.

The anterior ramus will then give off several branches including collateral intercostal, muscular, lateral cutaneous, and mammary branches. The posterior ramus gives off one branch, the spinal branch, that enters the vertebral canal. The superior phrenic arteries also arise from the thoracic aorta, later anastomosing with the pericardiacophrenic and musculophrenic arteries. The lowest branching arteries of the thoracic aorta are the subcostal arteries which will later give off a posterior branch. At the aortic hiatus of the diaphragm, the thoracic aorta will then become the abdominal aorta. The abdominal aorta continues downward, ending just above the groin when it divides into the two common iliac arteries.[1]

Embryology

The aorta develops during the third gestational week concurrently with the endocardial tube. During this early developmental stage, the primitive aorta demonstrates dorsal and ventral segments which are continuous through the first aortic arch. The first aortic arch will disappear during a subsequent developmental stage. The ventral segments of the primitive aorta will join and form the aortic sac while the dorsal segments will merge and create the midline descending aorta. There are six paired aortic arches which will develop between the ventral and dorsal aortae; these branches are the branchial arch arteries. Towards the end of aortic development, regression of the right dorsal aortic root along with the right ductus arteriosus will result in the formation of the physiologically normal left aortic arch.[1][2]

Blood Supply and Lymphatics

Blood enters the aorta, ejected from the left ventricle after being pushed through the aortic valve during contraction of the heart. Coronary arteries branch off of the first portion of the aorta to supply the heart muscle with oxygenated blood. The right coronary artery, the smaller of the two coronary arteries, arises from the right anterior aortic sinus that is present within the ascending portion of the aorta. After it branches off the aorta, the right coronary artery passes between the right auricula and the conus arteriosus. It then runs through the right side of the coronary sulcus onto the diaphragmatic surface of the heart and then heads left to the posterior longitudinal sulcus and finally down to the cardiac apex as the posterior descending branch of the right coronary artery. The marginal branch of the right coronary artery supplies oxygenated blood to the surface of the right ventricle of the heart as well as the right atrium and a portion of the left ventricle.

The left coronary artery branches off from the left anterior aortic sinus. This artery has two main branches, the anterior descending branch, and the circumflex branch. The anterior descending branch of the left coronary artery passes behind the pulmonary artery. This artery then travels between the pulmonary artery and the left auricula till it reaches the anterior longitudinal sulcus where it begins its descent to the incisura apices cordis where it can then branch to supply oxygenated blood to both ventricles. The circumflex branch of the left coronary artery provides branches to the left atrium and ventricle.[1]

Nerves

Baroreceptors and chemoreceptors within the aortic arch control homeostatic mechanisms via the autonomic nervous systems through communication with the medulla oblongata. The vagus nerve is responsible for carrying signals from the aortic baroreceptors. Several nerves pass downward along the left portion of the aortic arch. These nerves are the left phrenic nerve, the superior cardiac branch of the left sympathetic nerve, the trunk of the left vagus nerve, and the lower of the superior cardiac branches of the vagus nerve. The recurrent laryngeal nerve branches off of the left vagus nerve and loops around the aortic arch before traveling back towards the neck.

Muscles

The aorta is composed of vascular smooth muscle. Three different layers make up its walls. The first of those layers is the intima, which is the thin inner layer of the aorta. The other two layers are the middle elastic layer or the media and the outer fibrous layer or the adventitia.

Physiologic Variants

Variations in structure may occur during gestational growth as the aorta forms in conjunction with the endocardial tube around day 21 of development. Magnetic resonance imaging (MRI) is the currently accepted gold standard for imagining of the aortic arch. MRI is able to provide imaging of both the structural relationship between the aorta and its branches to the trachea and bronchi as well as the arterial branching pattern. Other imaging modalities that may be used for assessment of the aorta and which provide the ability to accurately and expertly assess aortic pathology include transthoracic echocardiography (TTE), computed tomography (CT), chest radiographs (CXR), transesophageal echocardiography (TOE), and invasive catheter angiography. Several anatomical abnormalities or variations are possible. One such variation, coarctation of the aorta, is a relatively common cause of congenital heart disease (CHD). This particular anatomical variation makes up 5 to 7 percent of all CHD cases. Coarctation of the aorta is an abnormality in which there is a narrowing of the aorta resulting in decreased blood flow. Presenting symptoms and time to diagnosis may vary from patient to patient based on the severity of the narrowing. The severity of the obstruction can range from a milder narrowing which often takes longer to diagnose to more severe blockages which will often be diagnosed in early infancy.

 Another potential anatomical abnormality that may occur is hypoplasia of the ascending aorta. This condition results in diminished blood flow and typically occurs alongside hypoplastic left heart syndrome (HLHS). HLHS is severe and if untreated may result in death. Patent ductus arteriosus is yet another potential pathological variation that may occur. It results when the ductus arteriosus fails to close following birth. The ductus arteriosus allows for the flow of blood between the aorta and pulmonary artery during intrauterine development and typically closes within a few days of delivery. An interrupted aortic arch is also a pathological variation that may occur with the aorta. In this abnormality, a portion of the aorta is missing which results in interrupted blood flow and is therefore of significant concern. This particular variant correlates with high rates of mortality. 

 Pulmonary sequestration is a rare anomaly that may occur involving the aorta in which a section of lung tissue obtains its blood supply from an abnormal source such as a systemic artery that originated from the aorta or one of its branches. Three variations occur within pulmonary sequestration. Those are extralobar, intralobar, and communicating bronchopulmonary foregut malformations, and usually, the type of variation centers on the development time of the accessory lung bud. Another potential abnormality, aortic ductus diverticulum, is an aortic variation in which there is an outpouching of the thoracic aorta. This condition is seen in about 9% of individuals and may be mistaken for an acute injury. Ductus diverticulum is most common at the aortic isthmus, distal to the origin point of the left subclavian artery. This variant appearance may resemble a pseudoaneurysm of the aortic isthmus, and therefore differentiation between the two is of particular importance.[1][3][4][5][6]

Surgical Considerations

The importance of the aorta regarding supplying oxygenated blood to the rest of the body combined with a large number of potential pathological variations make the aorta an important site for surgical considerations. Aortic aneurysms may occur, and when they do surgical repair will often be required. This repair may involve surgery performed through an abdominal incision or, depending on the severity, it could require removable of a section of the aorta that would then require surgical reconnection or grafting. Two more potential pathological conditions; aortic dissection and coarctation, may also require surgical intervention. Replacement of the aortic valve, the portion of the aorta that connects to the left ventricle, may be necessary under certain circumstances as well.[7][8]

Clinical Significance

The aorta is the primary vessel responsible for the flow of oxygenated blood out of the heart and into the rest of the body. It is the branching point for many subsequent arteries. Several clinically significant conditions should be taken into account when discussing the aorta. Those include coarctation of the aorta, aortic aneurysm, aortic valve stenosis, aortic dissection, aortic infection, as well as conditions such as atherosclerosis and hypertension which may negatively impact the health of the aorta.



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      Contributed Illustration by Beckie Palmer