The contraction of the muscular layer of the veins is also important, which in the veins of the lower half of the body, where the conditions for venous outflow is more complicated, is more developed than in the veins of the upper body. The reverse flow of venous blood is prevented by special devices of the veins – valves that make up the peculiarities of the venous wall. Venous valves consist of a fold of endothelium containing a layer of connective tissue. They face the free edge towards the heart and therefore do not prevent the blood from flowing in this direction, but keep it from returning back. Arteries and veins usually go together, with small and medium arteries accompanied by two veins, and large – one. Except for some deep veins, this rule excludes mainly superficial veins reaching the subcutaneous tissue and almost never accompanying arteries.
The walls of blood vessels have their own serving thin arteries and veins, vasa vasorum. They depart either from the same trunk, the wall of which is supplied with blood, or from the neighboring one and pass in the connective tissue layer surrounding the blood vessels and more or less closely connected with their outer sheath; This layer is called the vascular vagina, vagina vasorum. In the wall of arteries and veins there are numerous nerve endings (receptors and effectors) connected with the central nervous system, due to which the nervous regulation of blood circulation is carried out by the mechanism of reflexes. Blood vessels are extensive reflexogenic zones that play a large role in the neuro-humoral regulation of metabolism. Accordingly, the functions and the structure of the various departments and the features of innervation all the blood vessels in recent times have been divided into 3 groups:
1) the heart vessels that begin and end both circles of circulation, the aorta and the pulmonary trunk (i.e., elastic arteries), hollow and pulmonary veins;
2) the great vessels that serve for the distribution of blood throughout the body. These are large and medium sized extraorgan arteries of the muscular type and extraorgan veins;
3) organ vessels providing exchange reactions between the blood and the parenchyma of organs. These are intraorgan arteries and veins, as well as links in the microvasculature.