The heart continuously pumps blood around the body to distribute nutrients and remove waste. The blood is taken away from the heart in arteries under high pressure passes through meshes of tiny blood vessels in the tissues (eg leg muscles) and is returned to the heart via veins which are thin walled collapsible tubes normally under relatively low pressure. As veins are thin walled and collapsible the flow within them must rely on pumping from behind, rather than sucking from the heart in front as all this would achieve would be to collapse them. In the legs there are two main sets of large veins running parallel. The superficial (surface) system and the deep system. Blood in the superficial system flows into the deep system before being carried out of the leg up to the heart.