Arteriovenous anastomoses

Arteriovenous anastomoses - blood vessels that connect the arteries to the veins (Fig. 1). arterio-venous anastomoses are not usually included artificial (including trauma) arteriovenous fistula (arteriovenous aneurysm, the connection blood vessel with venous for connection of the apparatus "artificial kidney", and so on).
On the basis of functional and morphological features arterio-venous anastomoses can be divided into "direct" and "true." The structure of "direct" A.-century A. not differ from the structure of small arteries or veins. They are deprived of devices that regulate the width of the gap, due to which the number held by him of the blood is always about the same. Such anastomoses usually have a relatively wide lumen diameter (about 100 MK or more). They were found in different places, more often as the accidental discovery. Direct arterio-venous anastomoses often referred to as malformations and distinguish from the true A.-century A., having a special structure and function.
"True", or actually arterio-venous anastomoses, found in many organs and tissues (the small intestine, salivary gland cancer, tongue, lungs, heart, thyroid gland, and so on), some (nail phalanxes of fingers, penis, auricle) in particularly large numbers. They are almost always highly convoluted, with thick wall and the small diameter of the lumen (typically less than 100 MK).
In actual A.-century A. one can distinguish two segments - venous and arterial; the latter is usually a large part of the anastomosis, and only in him a well defined specific features of the structure. Actually A.-century A. divided into two main types, among which there are a variety of intermediate forms.
Type I - epitelialna, or pomocny,is characterized by a peculiar epitelialna transformation of muscle cells. Epithelioid cells are round or oval in shape, with light vitreous cytoplasm, located directly under the endothelium (Fig. 2). Inner elastic membrane is absent or reduced. Arteriovenous anastomoses of this type often branched, and sometimes form a complex tangle of branching of blood vessels, prisoners in a dense connective tissue capsule [so-called pomocy (see)]. Consider that glamurnye cells are able swelling to close the lumen of the anastomosis, and when obuchenie - release into the blood vasoactive substances type of acetylcholine.
Type II - the so-called trailing artery disease is characterized by the presence in an intim longitudinal muscle layer (rich elastic fibers), which is located between the endothelium and inner elastic membrane (Fig. 3). When these muscles are contracted with circular muscular middle layer of the shell, the closure of the lumen of the anastomosis.
Arterio-venous anastomoses, especially glamurnye, richly innervated. Consider that epithelioid cells together with related nerve endings play the role of a nervous system. The main function of A.-century A.- participation in hemodynamic regulation. Their openings are closed and opened (Fig. 4) in a certain rhythm (2-12 times in 1 min) according to the different physiological or pathological conditions. Thanks to this, arterio-venous anastomoses regulate the volume and blood pressure (with closed anastomoses local blood pressure rises) in the relevant parts of organs and tissues. In addition, the GA-century A. enrich oxygen (arterialization) venous blood and accelerate its movement (open anastomosis). In addition to the regulation of local blood circulation, all A.-century A. play a role in the General hemodynamics.
So, for example, paralytic collapse opening A.-century A. is one of the reasons of falling blood pressure. More importance A.-century A., especially skin, have in the mechanism of thermoregulation. A.-century A. glamurnogo type are involved in humoral and receptor regulation.
Regeneration A.-century A. occurs when the tumor vessels, and also due to the restructuring of conventional vessels. In some pathological processes (related to violation of capillary circulation, hypertension small circle), and with reduced functional load of the authority there is an increase in the number of A.-century A., with the help of which increases or, on the contrary, decreases blood flow to the affected organs.
Pathological changes arterio-venous anastomoses were studied enough yet. Well-known glamurnye tumors (see)arising from A.-century A. type pomozov. In certain pathological conditions were found inflammation and sclerosis A.-century A. (Fig. 5). Sclerosis and hyalinosis in the field of functional muscle or epithelioid elements also noted senile involution. In the result of pathological changes possible obliteration anastomosis or, on the contrary, its gaping and the inability to close, leading to the loss of their function.

Fig. 1. Arterio-venous anastomosis (arrow): 1 - Vienna; 2 - artery.
Fig. 2. Pomocny arteriovenous anastomoses: 1 - venous segment anastomosis; 2 - epithelioid cells; 3 - the endothelium.
Fig. 3. A longitudinal section through the closing of an artery in the lung. The vessel is almost closed: 1 - endothelium; 2 - longitudinal muscle layer.
Fig. 4. Indoor (left) and outdoor (right) arterio-venous anastomosis (aav): 1 - artery; 2 - Vienna (x-ray) .
Fig. 5. Atrophy and multiple sclerosis glamurnogo anastomosis in the nail phalanx of the thumb in diabetes.