Aorta

Aorta

The aorta is the largest artery in the body, an elastic artery about 2.5–3.5 cm wide at its root. It carries oxygenated blood from the left ventricle and, through its branches, supplies every systemic tissue. It is divided into the ascending aorta, the aortic arch and the descending (thoracic and abdominal) aorta.

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Overview

Root diameter≈2.5–3.5 cm

Abdominal diameter≈2 cm (aneurysm ≥3 cm)

Ascending aorta length≈5 cm

End of archT4/T5 (sternal angle)

Aortic hiatusT12

BifurcationL4

Course. The aorta begins at the aortic valve, at the level of the lower border of the left 3rd costal cartilage. It ascends inside the pericardium, arches backward and to the left over the root of the left lung, and descends through the posterior mediastinum. It passes through the aortic hiatus of the diaphragm at T12, runs down the abdomen just left of the midline and divides at L4 into the two common iliac arteries.

Windkessel function. The aorta is the prototype elastic artery. The media of the thoracic aorta contains several dozen layers of fenestrated elastic lamellae. In systole the aorta stretches and stores part of the stroke volume. In diastole it recoils, keeping blood flowing and maintaining diastolic pressure, which in turn drives coronary filling. With age the aorta stiffens, so systolic and pulse pressures rise (isolated systolic hypertension).

Relations of the arch. The left vagus and left phrenic nerves cross the left side of the arch. The left recurrent laryngeal nerve hooks under the arch, beside the ligamentum arteriosum. The trachea and oesophagus lie to the right of the arch, and the left brachiocephalic vein crosses in front of its three branches.

Location: It runs from the left ventricle (aortic valve) up through the middle and superior mediastinum, down through the posterior mediastinum and abdomen, to the bifurcation at L4.

Parts & subdivisions

Aortic root and ascending aorta
About 5 cm long and enclosed in the pericardium. It contains the three aortic sinuses (of Valsalva), and its only branches are the right and left coronary arteries. It ends at the level of the sternal angle (2nd right sternocostal joint).
Aortic arch
Begins behind the 2nd right sternocostal joint, arches backward and to the left over the left main bronchus and pulmonary trunk, and ends at the T4/T5 level (plane of the sternal angle). It gives off the brachiocephalic trunk, the left common carotid artery and the left subclavian artery. The ligamentum arteriosum is attached to its underside. Between the left subclavian origin and the ligamentum is a slight narrowing, the aortic isthmus, where coarctation and traumatic rupture typically occur. Baroreceptors in its wall send signals through the vagus nerve.
Descending thoracic aorta
Runs from T4 to T12 in the posterior mediastinum, starting left of the vertebral bodies and moving gradually toward the midline. The oesophagus lies first to its right and then in front of it. Its branches are the posterior intercostal arteries (3rd–11th spaces), subcostal arteries, left bronchial arteries and oesophageal, pericardial, mediastinal and superior phrenic branches. It leaves the thorax through the aortic hiatus at T12 with the thoracic duct and azygos vein.
Abdominal aorta
Runs from T12 to L4 in the retroperitoneum, to the left of the IVC. Its unpaired anterior branches are the coeliac trunk (T12), superior mesenteric artery (L1) and inferior mesenteric artery (L3). Its paired lateral branches are the inferior phrenic (T12), middle suprarenal (L1), renal (L1–L2) and gonadal (L2) arteries. Posteriorly it gives four pairs of lumbar arteries and the median sacral artery. It ends by dividing into the common iliac arteries at L4.

Anatomy

Branches: Ascending aorta: right and left coronary arteries. Arch: brachiocephalic trunk, left common carotid and left subclavian arteries. Thoracic aorta: posterior intercostal, subcostal, bronchial, oesophageal, pericardial, mediastinal and superior phrenic arteries. Abdominal aorta: coeliac trunk, superior mesenteric, inferior mesenteric, inferior phrenic, middle suprarenal, renal, gonadal, lumbar and median sacral arteries, ending as the common iliac arteries.
Histology: Elastic artery. The intima is relatively thick. The media contains many concentric fenestrated elastic lamellae alternating with smooth muscle cells. The adventitia is thin and carries vasa vasorum, which supply the outer part of the media.

Function

Conducts the entire left ventricular output to the systemic circulation.
Smooths pulsatile ejection into continuous flow and maintains diastolic pressure (Windkessel effect).
Senses arterial pressure through baroreceptors in the arch (vagal afferents).

Clinical significance

Aortic dissection

In aortic dissection, a tear in the intima lets blood track along a plane inside the media and create a false lumen. Risk factors are hypertension, Marfan, Loeys–Dietz and vascular Ehlers–Danlos syndromes, bicuspid aortic valve and cocaine use. Patients have sudden "tearing" chest pain that radiates to the back, unequal pulses or blood pressures in the arms, and a widened mediastinum on chest radiograph.

Stanford type A dissections involve the ascending aorta. They can cause aortic regurgitation, tamponade or coronary occlusion, and need emergency surgery. Type B dissections are confined to the descending aorta and are usually treated with blood pressure control, with endovascular repair (TEVAR) if complications arise.

Abdominal aortic aneurysm

An abdominal aortic aneurysm is a permanent dilatation of at least 3 cm, usually below the renal arteries. Risk factors are smoking, male sex, age and family history. Men are screened with an ultrasound scan at 65 in many countries. Repair (open or endovascular, EVAR) is offered when the diameter reaches about 5.5 cm in men, when the aneurysm grows faster than 1 cm a year or when it causes symptoms. Rupture causes back or flank pain, a pulsatile mass and shock, and has a very high mortality.

Coarctation and traumatic rupture

Coarctation is a narrowing near the aortic isthmus, close to the ligamentum arteriosum. It causes hypertension in the arms, weak and delayed femoral pulses (radio-femoral delay) and notching of the ribs on chest radiograph. The isthmus is also where the aorta tears in rapid deceleration injuries, because the mobile arch moves relative to the fixed descending aorta.

Mnemonics

Arch branches

ABC'S: the Arch gives the Brachiocephalic trunk, the left Common carotid and the left Subclavian.

Abdominal levels

"12 – 1 – 1/2 – 2 – 3 – 4": coeliac T12, SMA L1, renals L1–L2, gonadals L2, IMA L3, bifurcation L4.

Test yourself

At which vertebral level does the coeliac trunk usually arise from the aorta?

Answer: T12

The coeliac trunk arises at T12, just below the aortic hiatus. The SMA arises at L1, the renal arteries at L1–L2, the gonadal arteries at L2 and the IMA at L3, and the aorta bifurcates at L4.

A newborn with critical coarctation of the aorta collapses as the ductus closes. Which drug is given to reopen and maintain the ductus?

Indomethacin
Ibuprofen
Alprostadil (prostaglandin E1)
Paracetamol

Answer: Alprostadil (prostaglandin E1)

Prostaglandin E keeps the ductus open, so alprostadil (PGE1) is infused in duct-dependent lesions until surgery. Indomethacin, ibuprofen and paracetamol reduce prostaglandin synthesis and are used to close a PDA.

Which statement about Stanford type A aortic dissection is correct?

It is confined to the descending aorta and managed medically
It involves the ascending aorta and requires emergency surgery
It never causes aortic regurgitation
It is defined as an aortic diameter of 5.5 cm or more

Answer: It involves the ascending aorta and requires emergency surgery

Type A dissections involve the ascending aorta. They can cause aortic regurgitation, tamponade, coronary occlusion and stroke, and need emergency surgery. Type B dissections are confined to the descending aorta and are usually managed with blood pressure control or endovascular repair.