When blood flow is slow or sluggish, as can happen in poorly performing leg veins, your leg muscles compensate by helping to power blood up your legs to the heart. But if your movements are restricted for a period of time, such as sitting without moving for a long period of time, your ankles may swell and your legs ache. There may also be a low risk of a deep vein thrombosis or blood clot forming if you are at special risk.
The risk of DVT is low, but you can help reduce it by wearing Scholl Flight Socks or Scholl Flight Socks Sheer. Take every opportunity to leave your seat and move around, or flex your shoulders feet and ankles when this isn't possible. Drinking plenty of water and wearing loose clothing is a good idea.
It's also worth avoiding caffeine or alcohol, not allowing the backs of your calves to press up against your seat or using footrests that leave your feet dangling.