Blue-green lines that emerge directly under the skin are superficial veins. Depending upon the person’s body heat, the superficial veins can protrude and be quite prominent.
Veins are part of the blood circulatory system; they return blood to the heart from various parts of the body. Other veins are called deep veins, which are located close to an artery, are larger and cannot be seen on the surface of the skin.
Superficial veins are not close to any arteries, and are not as vital as deep veins because they carry much less blood. Superficial veins can actually be removed if they become varicose veins or are too prominent. This is a surgical procedure called vein stripping. Deep veins can take over the function of any removed or dysfunctional superficial veins.
The somewhat crucial superficial vein is the jugular vein which protrudes from both sides of the neck and carries a very high volume of blood from the head back to the heart. A useful superficial vein is the median cubital vein on both arms inside the crook of the elbow. Blood is drawn from them for blood tests. At times they are not prominent enough and need to be coaxed out through exertion.
As a major part of their function, superficial veins protrude after exertion or when the surrounding temperatures are hot as a means to control the body temperature. When the body gets too warm, the deep veins divert more blood to the superficial veins which are closer to the surface as a cooling method.