The veins form a convergent system of blood vessels, responsible for transporting blood from the venous end of the capillaries to the heart. For this reason all veins, with the exception of the pulmonary ones, carry deoxygenated blood rich in carbon dioxide. Proceeding from the periphery to the heart. heart, the blood flow flows into increasingly larger vessels, until it flows into the hollow veins directed to the right atrium of the heart, where the blood flowing from the coronary circulation also flows.
The blood coming from the supradiafframatic portion of the body flows into the superior vena cava, while that flowing from the underlying districts and from the lower limbs, flows into the inferior vena cava. From the right atrium, the blood is pushed into the ipsilateral ventricle and from there into the pulmonary artery, where it is enriched with oxygen; the return to the left atrium is entrusted to the pulmonary veins.
The veins are divided into superficial veins and deep veins. The first run in the subcutaneous, superficially to the fibrous bands that surround the muscles, so as to be clearly visible to the naked eye, especially during compression or physical efforts that make them turgid with blood. The deep veins, on the other hand, run under these bands in the muscular interstices and in the bone and body cavities, where - coupled with arteries and nerves - they form the so-called vascular-nerve bundles. In the peripheral bundles there are generally two veins for each artery, linked together by frequent anastomotic branches. Conversely, the vascular-nerve bundles next to the heart contain only one vein per artery. It is not surprising, therefore, that veins are numerically superior to arteries; their precise location, moreover, presents a greater degree of inter-individual variability. By examining the venous circulation, it is possible to recognize small connecting branches, called communicating or perforating veins, which connect the superficial and deep systems with a flow normally directed inwards.
Like the arterial ones, the walls of the veins are made up of three layers of tissue; while retaining a good expansive capacity, they are thinner and more inelastic than arteries of the same caliber. As proof of these characteristics, the superficial veins visible to the naked eye allow a glimpse of the blue tones of the dark blood that circulates within them, while on histological examination they appear flattened (unlike the arteries which maintain the cylindrical shape even when they are not perfused) . In fact, a lesion of a vein causes a "regular and continuous bleeding, while from an artery the blood - pushed by the rhythmic contractions of the heart - gushes out in a gushing way. Since the blood pressure inside the veins" it is low, the walls, although thin, present a low risk of injury. Beyond the greater thinness of the wall, the veins boast a larger diameter than the arteries, which is useful so that they can accommodate large quantities of blood while opposing a modest resistance; in fact, more than 65% of the total circulating blood is normally found inside the veins, which are therefore called capacitor (low resistance) vessels.
