Mans upright gait is the beginning of all venous problems. While oxygen enriched blood reaches the legs almost by itself thanks to gravity, the return flow is slightly more difficult. The veins have to pump the oxygen-deficient blood (low in oxygen) from below back up to the heart. When walking the calf muscles support the veins, press them together and thus the blood upwards. However, standing or sitting a lot makes the whole process more difficult for the veins. Blood can pool in the legs. Sales or office staff or travellers know all about this. In the evening legs are often swollen, tired and heavy.