Veins have a similar structure to arteries but have much thinner walls. They therefore distend and collapse easily, depending on the amount of blood flowing through them

Many larger veins in the lower part of the body contain valves to prevent blood flowing backwards. The valves are made of two flaps of tissue from the inner lining of the vein. When blood begins to flow backwards, the flaps fill with blood, bulge out and block the vein.

During exercise, when muscles are alternately contracted and relaxed, the veins are compressed and released. This forces blood back towards the heart.

Let's take a look at the major veins.