Veins store most of your blood and pump it home with valves
A vein returns blood toward the heart. Most systemic veins carry deoxygenated blood, though pulmonary and fetal veins are oxygen-rich exceptions. Compared with arteries they have thinner walls, wider lumens, and—holding nearly seventy percent of circulating volume—earn the name capacitance vessels.
Sizes run from post-capillary venules ten to thirty micrometres across, through muscular venules and small veins, up to medium and large trunks that empty into the superior and inferior venae cavae. Walls still show three tunics, but the media is thin because systolic pressure is low. Medium and large veins carry one-way bicuspid valves of intimal leaflets that stop gravity-driven reflux; skeletal muscle pumps in the legs squeeze intramuscular veins forward, and perforators shut so blood cannot spill back into superficial channels.
Deep veins travel with arteries, sometimes as paired venae comitantes that hitch a ride on arterial pulsation and exchange heat in counterflow. Superficial veins lack arterial partners and include spider veins half a millimetre wide; perforators link the two systems in the limbs. Vein walls thicken in the legs more than the arms, and exact courses vary person to person—unlike the more stereotyped arterial map.
Skin-blue appearance is a trick of scattering and cortex, not turquoise blood; venous blood itself is dark red. Special plexuses such as Batson’s valveless vertebral network can seed metastasis, while the eustachian valve at the inferior vena cava–atrium junction is mostly an embryonic leftover. From microcirculation to cavae, veins are not passive return pipes but a regulated reservoir that stores volume, meters pressure, and uses valves plus muscle to finish every heartbeat’s loop.
Valves can be counted with surprising precision. The thigh stretch of the great saphenous vein holds anywhere from one to seven, the segment below the knee two to six, and the marginal veins of the foot one to four. A terminal valve where that vein meets the femoral vein, plus a preterminal one just below where its tributaries enter, blocks reflux; when they fail, varicose veins commonly follow. Each valve is a pair of collagen-stiffened leaflets whose endothelial cells line up with the flow on one face and across it on the other. Meanwhile about three-quarters of all venous blood sits not in the big trunks but in small veins and venules.
Source: Vein
