Your leg muscles function like a second heart to pump fluid from your legs to your heart. When your leg muscles relax, the veins near them fill up with blood. When your leg muscles contract, they compress the veins and squeeze blood up toward your heart.
When you stand still, your heart has to work very hard to pump blood against gravity from your feet to your heart. When your feet are above your heart, gravity works with you to help blood and fluid return to the heart. Eight hours of standing or sitting causes your feet to swell up to more than 110% of their size. This can make your shoes feel tight and your feet hurt.
The best way to prevent swelling is to elevate your feet. The next best way is to move your feet and toes frequently while you are sitting or standing. This can reduce swelling by more than 50 percent and will usually prevent the pain that it causes. If your feet still swell, check with your doctor. You may have a more serious cause, or you may need diuretics or compression stockings.