Veins in the feet can look bigger when you run, but healthy veins won't expand since their blood volume and pressure fall during exercise. It's all part of a natural process and not anything to worry about. When you exercise, it is the blood pressure in your arteries that increases, forcing the plasma or liquid portion of the blood into the area surrounding the muscles.
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Veins Appear Bigger
The leaked plasma causes the muscle tissue to harden and swell. Greater physical exertion will cause the veins to become more noticeable as they are pushed closer to the skin, but their appearance will gradually return to normal after exercise. However, in the case of varicose veins, which have faulty valves, blood pools and will cause the veins to expand. If you have varicosities, less vigorous exercise is recommended to boost your circulation; high-impact exercise like running is not recommended.