In a normal person, the heart pumps arterial blood that is full of oxygen and nutrients, out into the body, limbs and head through the arterial system. This is best considered when the patient is lying down.
Once the blood has travelled through the arteries into the capillaries (which lie in all of the tissues of the body), the oxygen and nutrients are transferred to the tissues and are replaced with waste products, including carbon dioxide, water and urea. The venous blood at the ankle just after going through the capillaries has just enough pressure left in it to push it back to the heart. Therefore if we spend all our time lying on our backs, the veins would have very little work to do (as shown above).
When we stand up, the pumping action of the heart on the arterial blood flowing to the legs is enhanced by gravity.
Unfortunately, although gravity helps the arterial blood whilst standing, gravity works directly against the venous blood returning to the heart through the veins. As such, when standing, blood that has gone through the capillaries in the feet and ankles only has enough pressure to get back to the ankles and lower leg, and no further.
It takes another “pump” like an extra “heart” to pump the blood from the ankles back up to the heart where the waste products can be removed through the lungs and kidneys, and the blood can be loaded up again with oxygen and nutrients to start the process once again.
Therefore the veins work by movement of the legs, “pumping” the blood up and out of the veins.
The veins themselves do not do any pumping. They are passive blood vessels which are very elastic and can distend when full with blood, but can be easily compressed allowing blood to be pushed out of them.
In the legs, the deep veins are encased in muscle and so any movement, no matter how slight, results in compression and hence pumping of the blood in the veins. In the leg, there are actually three muscle pumps and also a foot pump that works in a different way, all of which are coordinated with the act of walking. Again if you would like to know more, please see The College of Phlebology’s textbook “Understanding Venous Reflux – the cause of varicose veins and venous leg ulcers
” where there is an easy to follow explanation of the whole of the series of leg pumps and how walking gives optimal venous pumping.
However, as with all pumps, the movement supplying pressure to the blood to make it move is only one part of the story. As can be seen from the animation above, if pressure is placed upon a vein which is just a simple tube, blood would flow both upwards and downwards and not in any ordered pumping fashion.
What makes veins in the systemic circulation special is that they also contain “valves”. These valves are pairs of membranes on the inner aspect of the vein wall, that acts like a pair of pockets pointing upwards. When blood is being actively pumped upwards, the valve leaflets are pushed against the wall by the flow of the blood, allowing the blood to flow upwards without any hindrance. However when the pumping stops, and the blood starts to fall backwards with gravity, the valve leaflets are forced open by the reverse blood flow, catching the blood in the “pockets” which opens the valves further, preventing any backward flow down the vein.
It is thanks to the pumping of blood upwards during active contraction of the muscles, followed by the closure of the valve stopping the blood falling back down when the muscles relax that make sure that blood only flows one-way in veins and continues upwards towards the heart.