1. In the veins, blood flows in most parts of the body (trunk and limbs) against the direction of gravity and therefore slower than in the arteries. Its balance in the heart is achieved by the fact that the venous bed in its mass is much wider than the arterial one. The greater width of the venous bed compared with the arterial is provided by the following anatomical devices: a large caliber of veins, a large number of them, paired accompaniment of arteries, the presence of veins not accompanying the arteries, a large number of anastomoses and greater density of the venous network, the formation of venous plexuses and sinuses, the presence of portal system in the liver. Because of this, venous blood flows to the heart through three large vessels (two hollow veins and the coronary sinus, not to mention the small veins of the heart), while the about one pulmonary trunk.
2. The deep veins accompanying the arteries, i.e., the vein satellites (venae commitantes), in their distribution obey the same laws as the arteries they accompany (see “Regularities in the distribution of arteries”), while most of them accompany the arteries in double number. Paired veins are found mainly where the venous outflow is most difficult, that is, in the extremities, since such a structure has developed even in four-legged animals, in which both pairs of extremities occupy a sheer position and the torso is horizontal.
3. Accordingly, the grouping of the whole body around the nervous system deep veins are located along the nerve tube and nerves. Thus, parallel to the spinal cord is the inferior vena cava, and each segment of the spinal cord corresponds to segmental veins, for example, vv. lumbales and rr. spinales.
4. According to the division of the body into the organs of plant and animal life, the veins are divided into parietal – from the walls of the body cavities and visceral – from their contents, i.e. from the inside.
5. Most of the veins are located on the principle of bilateral symmetry.
6. The veins of the trunk walls retain a segmental structure.
7. Deep veins go along with other parts of the vascular system – arteries and lymphatic vessels, as well as nerves, participating in the formation of neurovascular bundles.
8. Veins also go according to the skeleton. So, along the spine is the inferior vena cava, along the ribs – intercostal veins, along the bones of the limbs – the veins of the same name: shoulder, radial, ulnar, femoral, etc.
9. The veins travel along the shortest distance, that is, approximately in a straight line connecting the place of origin of this vein to its confluence.
10. Superficial veins lying under the skin accompany the skin nerves. A significant part of the superficial veins form subcutaneous venous networks that have no relation to either the nerves or the arteries.
11. Venous plexuses are found mainly on the internal organs, which change their volume, but are located in cavities with unyielding walls, and facilitate the outflow of venous blood with an increase in organs and compression of their walls. This explains the abundance of venous plexuses around the pelvic organs (bladder, uterus, rectum), in the spinal canal, where the pressure of the cerebrospinal fluid constantly fluctuates, and in other similar places.
12. In the cranial cavity, where the slightest obstruction of the venous outflow affects the brain function, there are, in addition to the veins, special devices – the venous sinuses with unyielding walls formed by a hard shell. Therefore, they lie mainly at the site of attachment of durae matris processes to the bones of the skull (sutures of the integumentary bones and sinous bones of the sinuses).
13. Special devices include veins located in the channels diploe – venae diploicae.