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The Aorta
Written by Tim Sheppard MBBS BSc. Created 20/6/10; last updated 19/5/11

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What is the aorta?

The heart pumps blood around the whole body, which travels through blood vessels to arrive back in the heart. While it continues its important job of transport, it needs a decent carriageway to travel along. If its only route is a narrow English country lane, not very much traffic is going to be able to get along it. Fortunately there are some pretty major highways that the blood travels along before it gets to its destination, so that a large volume of blood can get through at any one moment in time. The biggest of these is the artery closest to the heart, the aorta.

The aorta, then, is the first and largest artery leaving the left side of the heart. Like other arteries, the middle layer of the wall of this artery (the tunica media) is thick, giving it the strength and elasticity which it needs to recieve blood at high pressure from the heart and send it on its way.

In fact, the elasticity of the aorta is an incredibly importance feature because it helps to propel blood forward. As the heart pumps blood firmly through this vessel (during systole), the walls stretch from all the pressure. When the heart relaxes (diastole), the blood is no longer being propelled forwards. The aortic valve stops blood from flowing backwards, and as the stretched wall of the aorta shrinks back, blood is pushed forward through the body - known as the Windkessel effect.

What is the anatomy of the aorta?

In many ways the aorta doesn't "finish" because it simply divides, and there is no boundary between where it ends and the next artery begins. It starts at the opening of the left ventricle, from which blood is pumped out of the heart through a valve - rather imaginatively named the aortic valve because it leads to this artery. The artery loops up and then curls behind the heart to descend through the thorax (or chest) into the abdomen. The first part is called the ascending aorta, the loop is called the arch of the aorta, and it becomes the descending thoracic aorta and abdominal aorta as it reaches the appropriate regions. Throughout this journey it throws off branches, which we shall come to in a moment, but eventually it gives up and decides to split entirely in two, separating into the right and left common iliac arteries, which supply blood to the lower part of the body and each leg.

Strictly speaking, the very first branches that come off the aorta are the right and left coronary arteries. These supply the heart, so they're extremely important - and it is blockage of one of these that causes a heart attack. These arteries come off the aorta right at its root, through two of what are known as the coronary ostia (which lie behind the flaps of the aortic valve), and blood flows through them when the heart is relaxed (i.e. in diastole).

Assuming all things to be normal, the next branch that comes off the aorta arises at the start of the arch of the aorta, and is called the (right) brachiocephalic artery. This then divides to become the right subclavian artery (leading to the arm) and the right common carotid artery, leading to the head. On the left side, because there is more room at this point, the left common carotid and left subclavian arteries come off separately. There is therefore no left brachiocephalic artery. On some rare occasions, when the heart is on the wrong side of the body (the right side - known as dextrocardia), which will usually be no problem, but you'll have a left brachiocephalic artery and no right brachiocephalic artery (with separate subclavian and common carotid arteries on the right).

Because the lungs get most of their blood from the right side of the heart (through the pulmonary artery), there is little else for the aorta to provide blood for before it reaches the abdomen. Having provided the major branches to the head and arms, a few smaller branches provide oxygenated blood to the lungs and oesophagus (food pipe) before the aorta drops through the diaphragm into the abdomen.

It can be useful to know how far along the branches of the aorta leave it as it goes along, but unfortunately when it is traveling in the middle of the body, it may be difficult to find any useful landmarks. The best way to show what level you're talking about is usually by talking about which of the vertebrae you're level with. For instance, the aorta plunges through the diaphragm at about T12 (the last thoracic vertebra).

Having emerged through the diaphragm, the aorta starts giving off numerous branches to supply blood to the surrounding organs desperately thirsty for oxygen and nutrients. A relatively small branch to the diaphragm (called the inferior phrenic artery) comes before the caeliac trunk, all at T12. The caeliac trunk is called a trunk rather than an artery because it quickly divides into three major arteries to supply organs at the top of the abdomen: the hepatic artery to supply the liver, the gastric artery to supply the stomach, and the splenic artery to supply the spleen.

At approximately L1 you get the first of the arteries supplying the intestines - the superior mesenteric artery. Before the other main supply to the intestines you have small branches to the adrenal glands - the middle suprarenal arteries. Then, two of the most important arteries in the body - the left and right renal arteries, carrying a fifth of the blood that has left the heart to the kidneys. Just below these, at about L2, you have the gonadal arteries to the reproductive organs - ovaries in women, testes in men. The left gonadal artery comes from the aorta; the right gonadal artery arises from the right renal artery.

Accompanying the inferior mesenteric artery supply to the intestines at L3 are small branches supplying other parts of the abdomen and pelvis - the eight lumbar arteries (four from each side) and the median sacral artery.

Finally the aorta splits at about L4 into the left and right common iliac arteries.

What are the diseases of the aorta?

Lots of things can potentially go wrong with this blood vessel, most of which are beyond the scope of this little summary, but the main problems are listed below.

Aortic aneurysm - a swelling or aneurysm of the artery can happen anywhere along its route. There are several different types of aneurysm, but with the aorta, an aneurysm usually happens in the descending section, below the diaphragm. These swellings tend to occur over a long period of time, so they are more common in people who are middle-aged or older, and if they remain stable they don't tend to cause any problems. The problem is that they continue to grow.

Although the aorta is swollen, blood is still only going to travel through the centre of the artery, so a blood clot tends to form in the outer portions of the artery. Normally this is a stable clot and segments don't tend to fire off into the legs. However, as the swelling grows, the walls of the aorta get thinner and thinner, and eventually the swelling will pop. A burst aortic aneurysm is an absolute emergency, and if you're not already in the operating theatre when it pops, you are unlikely to survive.

Fixing an aortic aneurysm can be done with open surgery, or by inserting a stent into the aorta - a hollow tube that replaces the thin walls of the artery and which shouldn't swell up. Because you're dealing with such an important artery, any surgery in this area is dangerous and so it will only be carried out if the aneurysm is considered to be particularly dangerous - e.g. larger than 5.5cm in diameter.

Aortic dissection - when the wall of an artery is damaged in some way or another, it should usually repair itself. Unfortunately for the aorta, the blood is at such high pressure, it might take advantage of a small break in the lining of the wall (the intima). Instead of just traveling through the centre of the tube (the lumen), it might start to go into the wall of the artery. As the blood enters, this starts to split the wall of the artery, forcing the intima into the lumen and thereby blocking the aorta. As this split spreads, it may start to interupt one of the branches of the aorta. If it blocks one of these branches, it is a disaster, so many aortic dissections need treating as an emergency.

Aortic dissections are probably most concerning if they affect the early parts of the aorta, because if they block off the coronary sinus (i.e. they block the coronary arteries), they would cause a heart attack. More commonly, if they block the brachiocephalic artery or left common carotid artery, they will cause a stroke - or, quite possibly, death. Dissections affecting other parts of the aorta (or other arteries) may cause damage to other organs, but none of them are likely to cause death so quickly, but even getting rid of the kidneys, which are very important, wouldn't cause death as quickly as blocking blood supply to the brain.

Coarctation of the aorta - amongst many heart problems that can affect babies when they are born is a coarctation - simply a narrowing of the aorta. This makes it more difficult for blood to get to any arteries that branch off the aorta after the narrowing. The seriousness of the condition therefore depends on the position of the narrowing and how narrow it is. If it happens before the arch of the aorta, it can prevent blood from getting to the brain, and is therefore potentially life threatening. If it happens after the arch, it can still cause a problem, but may not be discovered until adulthood.

Connective tissue diseases of the aorta, such as Marfan syndrome or Ehlers-Danlos syndrome lead to weakening of the walls of the artery. Even this can lead to direct problems like widening of the artery, but is more of a problem because it makes it more likely that the aorta will develop complications - such an aneurysm, or dissection. If the widening (dilatation) happens at the start of the aorta, it may stop the cusps of the aortic valve from meeting properly in the middle, leading to aortic regurgitation.

Aortitis is a rare complication of ankylosing spondylitis, and is (as the name suggests) an inflammation of the aorta. It can also be caused by an infection, and requires treatment to make sure that it continues to function properly. It's easy to think of the aorta as rather simple, but it has a vitally important function in getting blood from the heart to the right part of the body, and any problem with it being able to achieve this can cause problems for any number of parts of the body.

Further Reading