What causes feet and lower legs to swell when flying?

Swelling: If you sit through the whole flight, you might be blocking the venous return of your lower extremities specially with varicose veins. Standup, strech, walk on the isle.

Blood in legs: The valves in your leg veins are not pumping the blood back up. This is the start of Congestive Heart Failure. It is highly suggested to use compression stockings on your legs during these flights. In addition, your physician may suggest that you wear them even in your normal activities to assist in not accumulating uncirculated blood.