Veins are the system of tubes that recycle blood from the entire body and transport it back to the heart. Starting from the capillary network, the venous lumen is relatively large, less elastic, thin walled, and localized compression can easily lead to obstruction of blood circulation. Veins contain venous valves, the main role is to prevent the backflow of blood, body circulation within the veins transported as venous blood, venous blood within the carbon dioxide content is relatively high, the amount of oxygen is small, dark red, in the superficial location of the body surface found in the veins for the purplish-blue, that is, blue veins. The pulmonary veins have a special place is that blood is exchanged from the lungs for gases, and when it is transported from the pulmonary veins back to the heart, the blood is rich in oxygen, and the blood in the pulmonary veins is bright red in color. The subcutaneous veins will be more visible in people who regularly participate in physical activity sports exercise.