How do veins work

Summary of the circulatory system
Proper circulation is essential for maintaining a healthy organism. The blood transports oxygen from the lungs to every cell in your body and takes care of the overall metabolism in our body.
Veins are an essential part of this system in the human body.The circulatory system  includes venous, arterial and lymphatic part and also the heart (perfect pump). Each of them is perfected structurally and functionally. Here are some characteristics of the blood circulation and the role of the veins in it:

•  Veins return blood from the periphery to the heart, which sends it using the arteries to every organ in the body.
•  Venous blood is low in oxygen and rich in waste, toxic products of the cell metabolism,    unlike the arterial which is rich in oxygen and building nutrients.
•  The venous system is closely connected with all the other systems and has perfect self-regulation.
•  The venous system starts from the smallest vessels in the tissues (venules with size fractions of a millimeter), which merge successively into larger and larger in caliber vein, reaching 12-18mm. There is an amazing resemblance with the formation of rivers in nature. 
•  The speed and pressure of blood in the veins are much lower than those in the arteries.
•  The venous system of the limbs has two parts - deep and superficial. The first is located close to the bone structures between large muscle groups. Through it flows 95-97% of venous blood in the limb. The superficial part is located under the skin and assumes 3-5% of the blood. Between the two parts exist linking (perforator) veins.
•  The veins have no pulse, the blood is propelled by pressure which is generated from the outside (from neighboring muscles, so called. Muscular pump) and not as in the arteries of the muscle layer of the vessel itself.
•  Only veins have valves which ensure unidirectional flow of blood. The valves vary in number and placement in the different types of veins.
•  Deviation in either blood composition and structure of the venous system are the cause of inflammation and thrombosis.
• How does blood flow in different situations?The heart pumps blood under high pressure in the arteries who distribute it to the limbs. There oxygen and nutrients are transported to the tissues and carbon dioxide and waste products are evacuated and taken up by the veins. Unfortunately, there is no "heart" in the legs to pump the blood back, so a different method is used for the return of blood.

When a person is lying down the pressure in the veins is enough so that the blood can flow continuously and smoothly to return to the heart.

But when you are standing the situation is different. The pressure in the ankle increases due to hydrostatic (gravitational) pressure. It helps blood flow in the arteries but disturbs the venous flow. This is avoided by the so called "muscle pump of the leg". What is it? When you walk your leg muscles contract, which  presses and squeezes blood from the veins located between them. This is followed by relaxation of muscles, filling the veins and prepairing for a new cycle. Venous valves direct blood in the right direction and prevent its return.
Imbalance in this perfect mechanism is the main cause of nearly all venous problems
When blood circulation is not functioning well complications and diseases  appear: varicose veins, thrombosis, teleagiectasia, reticular veins, trophic ulcers. These diseases are often quite painful and disrupt motor function of the affected limb.

Fortunately, the rate at which modern medicine develops makes, many quick, painless and effective treatment methods possible.