The abdominal aorta is the descending portion of the large body artery below the thoracic aorta. The abdominal aorta begins at the level of the diaphragmatic hernia and extends to branch into the two major iliac arteries at the level of the fourth lumbar vertebra. The two larger renal arteries and a number of smaller arteries branch off from the abdominal aorta, which performs part of the aorta’s wind chamber function, to supply the internal organs located within its catchment area and the periphery.
What is the abdominal aorta?
The abdominal aorta represents a segment of the descending great artery of the body (descending aorta). It begins at the lower end of the so-called thoracic aorta (aorta thoracica) at the opening through the diaphragm (hiatus aorticus), at the level of the twelfth thoracic vertebra. The abdominal aorta terminates at the level of the fourth lumbar vertebra at the bifurcation of the abdominal aorta (bifurcatio aortae) into the two iliac arteries (arteriae iliacae communes). Overall, the abdominal aorta forms an anatomical and functional unit with the other segments of the body’s aorta. In the first third, the two large renal arteries (Arteriae renales) branch off, so that a distinction is made in the abdominal aorta between the section above (suprarenal) and below (infrarenal) the branch of the renal arteries. In addition to the two renal arteries, many other arteries branch off from the abdominal aorta to supply the internal organs and peripheral regions.
Anatomy and structure
Immediately below the passage through the diaphragm, two relatively thin branches branch off from the abdominal aorta to supply the lower diaphragmatic regions. At about the same level, the common arterial trunk (truncus coeliacus) arises anteriorly toward the abdominal cavity, which immediately thereafter divides into three arteries to supply the spleen, liver, and stomach. In the further course of the abdominal aorta, further paired or unpaired arteries branch off to supply the viscera or peripheral regions. The largest paired branches are formed by the two renal arteries (Arteria renalis dexter and sinister). As with the other major arteries, the abdominal aorta is found to have a three-layered wall structure. The inner layer, the tunica intima or simply intima, is composed of endothelial cells that are interdigitated and form a single-layer squamous epithelium. Outwardly, there is a thin layer of connective tissue that demarcates the intima from the middle layer, the tunica media or media. It consists of smooth muscle cells, which are mostly surrounded by blood vessels and sometimes also by lymphatic vessels in a spiral pattern. In addition, elastic fibers, collagen and connective tissue cells are found in the media, marking the boundary with the outer wall layer, the tunica adventitia. The tunica adventitia or adventitia is formed by a relatively thick layer of connective tissue cells reinforced by collagen and elastic fibers. The outer wall layer of the abdominal aorta houses the vascular systems necessary for metabolic supply and disposal of the abdominal artery and nerve fibers to control the lumen of the abdominal artery.
Function and Tasks
As a segment of the great body artery, the function and tasks of the abdominal aorta are congruent with those of the aorta as a total system. It focuses on two main tasks of smoothing peak blood pressure and distributing oxygen-rich arterial blood to all organs and tissues. The elasticity or distensibility of the aortic walls in conjunction with their controllable contractility ensures the smoothing of systolic blood pressure peaks caused by contraction of the ventricles. Of particular importance is the maintenance of diastolic “residual pressure” when the ventricles relax during diastole. A minimum diastolic blood pressure ensures that the small arteries, arterioles, and arterial capillaries are supplied with a continuous blood flow and do not irreversibly collapse and stick together. The ability to smooth peak blood pressure is often referred to as the Windkessel function because the aortic wall contracts again during ventricular diastole and provides lumen reduction to maintain blood pressure. This is a process that is partly passive, but also contains active elements via hormonally controlled contractions of the vascular muscle.The second task of the abdominal aorta, the distribution of oxygen-rich arterial blood to organs and tissues, is performed passively via branching arteries. Their dimensions are adapted in each case to the requirements.
Diseases
The most common complaints associated with the abdominal aorta are caused by altered elasticity of the vessel wall or by localized narrowing or widening of the cross-section of the abdominal artery. Decreasing elasticity of the aortic wall, also known as arteriosclerosis, is the result of deposits (plaques) of various substances in the arterial wall. When the plaques reach a certain size, they protrude into the lumen of the arteries. In addition to hardening of the aortic wall, they then lead to a local bottleneck in the artery, which can develop into a total occlusion, or infarction. In rarer cases, a dangerous bulge, an aneurysm, can form in the abdominal aorta, which can have very different causes. In the early stages, it causes little discomfort, so such aneurysms tend to be discovered by chance. The danger lies in the possible rupture, a bursting of the aneurysm, which is accompanied by violent internal bleeding. Another problem can occur when the internal wall of the aorta is ruptured because hemorrhage can occur between the intima and media via the rupture, resulting in aortic dissection, a separation between the intima and media (aneurysm dissecans aortae). In rare cases, the aorta may be affected by genetic abnormalities. Autoimmune diseases such as Takayasu’s arteritis are also known to be associated with the abdominal aorta.