Histology and tissue (microscopy)
Histologically there are three layers: 1. intima: The intima is the innermost layer of the aorta and consists of the endothelium and a subendothelial layer. On a basal lamina there are unicellular layers of so-called endothelial cells, which have a negative charge at the tip (apical) due to a glycocalyx (sugar connected to the cell membrane). These cells are flat and have their longitudinal axis parallel to the bloodstream.
The individual cells are connected by dense membrane protein compounds (e.g. tight junctions, gap junctions, desmosomes). This seals the space between the cells, regulates the paracellular transport (cells can leave the blood system without damaging the cell wall!) and ensures the polarity of the cells.
The endothelium forms a barrier of the aorta through which the exchange of substances with the tissue takes place. It also plays an important role in blood clotting and inflammatory reactions (adhesion of platelets and white blood cells), as well as in the regulation of vessel width. The subendothelial layer of the aorta consists of the extracellular matrix.
It contains, for example, collagen and elastic fibres, collagen (type IV), microfibrils, fibrillin, proteoglycans, etc. . This layer is the site of vascular calcification (atherosclerosis).
2. media (tunica media): This middle layer consists of elastic and collagen fibres and mainly of (smooth) muscle cells, which are arranged in a spiral or ring shape and regulate the vascular width. 3. adventitia (tunica externa): This outermost layer of the aorta consists mainly of connective tissue and anchors the vessel in its surroundings. It also contains vessels for the blood supply (vasa vasorum) and nerve vessels.
Between the intima and the media and between the media and the adventitia there is another Membrana elastica (internal and external). This is an elastic lamella. The aorta belongs to the arteries of the elastic type. In this type of vessels the media is particularly thick and contains many elastic fibres, which is important for the function of the aorta.
Diseases of the aorta
Aortic valve stenosis is the almost complete closure of the aortic valve. The stenosis can be caused by a congenital malformation, arteriosclerosis, rheumatic inflammation or endocarditis (inflammation of the inner lining of the heart), caused by a bacterial infection. The stenosis leads to a pressure load on the left ventricle.
The blood in the ventricle can only be ejected against a higher pressure because the heart valve can no longer open completely. Compensatory muscle hypertrophy (the heart muscle becomes larger) of the left ventricle occurs, which has other consequences, such as a higher heart beat rate due to a higher oxygen demand for the increased muscle mass. Symptoms are absent for a long time, and symptoms such as tiredness, dizziness or dysrhythmia appear late.
Aortic valve stenosis is treated from a pressure gradient of more than 50mmHg between the left ventricle and the ascending aorta or in symptomatic patients. Aortic valve insufficiency is the inability to close the aortic valve. This can be caused by an increase in the connective tissue of the valve (fibrosis) and a consequent shrinkage of the valve, as is often the case with rheumatic inflammation.
This dilatation (enlargement) can be caused by an increased blood volume in the left ventricle, whereby the heart initially reacts with an increase in the stroke volume and a dilatation of the ventricle (chamber) and later also an increase in muscle mass. This increase in volume load is defined and described by the Frank-Starling mechanism. Aortic valve insufficiency is treated by surgery if the patient shows limited ability to bear weight, if the insufficiency is severe or if the volume in the left ventricle is significantly increased.
This topic may also be of interest to you: Heart valve diseasesAortic rupture is caused by increased mechanical stress from the blood flow, as well as a pre-damaged wall. Depending on which wall layer is ruptured, the lumen may be displaced, as in aortic dissection, or free bleeding may occur. A covered rupture can occur, whereby the exit of blood from the aorta is stopped by the peritoneum and blood can seep in over a few days.
Patients with a rupture of the aorta experience sudden crushing pain in the back and/or abdomen, often accompanied by symptoms of shock with a drop in blood pressure or fear of death, as well as subjective shortness of breath or undersupplied blood to the lower extremities. If a tear in the aorta remains undetected and is not a covered rupture, death occurs within minutes. A covered rupture is also an emergency indication and must be operated on immediately if it is discovered in time.
This topic might also be of interest to you: Calcifications in the abdominal arteryAn aortic aneurysm is a localized dilation of the aorta. A distinction is made between a real aneurysm (aneurysm verum), where all wall layers are affected, and a false aneurysm. In the fake aneurysm, only the outermost wall layer, the adventitia, is affected.
False aneurysms can take different forms, such as sacciformis or fusiformis. An aneurysm is caused by the weakening of the elastic force of the media (middle wall layer of the vessel), as a result of which the vessel can no longer withstand intravascular pressure and “bulges”. There are many causes for the development of an aneurysm.
For example, arterial hypertension (high blood pressure), arteriosclerosis or a congenital weakness of the connective tissue (e.g. Marfan syndrome) can be responsible for this. Symptoms such as pain in the back, a feeling of pressure or subjectively perceived shortness of breath may occur, but are not specific to an aortic aneurysm. For diagnostic clarification, an imaging procedure such as computer tomography (CT) or magnetic resonance imaging (MRI) can be considered.
The critical diameter of 5 cm for the ascending aorta and the aortic arch or 6 cm for the descending aorta is an indication for surgery. But even if the aneurysm grows more than 1cm in 3 months, surgery should be considered. Often a stent is implanted during surgery on the descending aorta, as long as no other outgoing artery is displaced during the procedure.
Aortic dissection is the splitting of the wall layers of the aorta. The starting point of the splitting of the wall layers is the tunica intima, the innermost layer of the aorta where the blood has direct contact. Bleeding occurs between the tunica intima and the media, which is the subsequent wall layer.
The bleeding causes the lumen to shift, resulting in a “true lumen” and a “false lumen”. Lumen refers to the hollow space in a vessel. The tearing of the intima and the formation of the “false lumen” can lead to the displacement of the true lumen.
Entry is the tear in the intima of the aorta, reentry is the point at which the blood from the false lumen passes back into the true lumen. Aortic dissection can be classified according to the Stanford and DeBakey classification. Both classifications describe the location of the dissection. Typical symptoms of aortic dissection are a stabbing pain radiating into the shoulder and/or a so-called pain of destruction, in which one can also feel fear of death. The dissection is treated in a similar way to the aneurysm by means of surgery using a tubular prosthesis or stent.