Your veins and arteries are part of your circulatory system. Arteries pump blood around your body. Once oxygen and nutrients are delivered, your blood flows into your veins, some of which are just below the skin. 

The forward flow of blood is ensured through:

• One way valves in the veins, preventing back flow
• Your calf muscles squeezing the  blood toward your heart when you move about
• Your arteries transmitting energy to the veins to propel blood towards the heart
• Elasticity, as veins stretch and contract  to help with upward blood flow

However, the effectiveness of these valves and the elasticity of your vein walls can be damaged or reduced, which results in the back flow and pooling of blood in your veins. Your veins then engorge and dilate. This is called venous insufficiency. In the larger vessels venous insufficiency can lead to varicose veins, whilst in the smaller more superficial vessels and capillaries reticular and spider veins can result.

Commonly, they occur in the legs because the veins in your lower extremities have to overcome gravity and endure the most pressure to move blood back to your heart. If the pressure is stronger than the one-way valves and vein wall tone, varicose and spider veins develop.