Why do your feet swell when you sit down for a while?

Feet can swell when you sit for extended periods due to fluid accumulation in the lower extremities, a condition known as edema. When sitting, gravity causes blood and fluids to pool in the legs and feet, especially if you're in a fixed position without much movement. Poor circulation and prolonged pressure on certain areas can exacerbate this swelling. To alleviate it, it's helpful to move around periodically or elevate your feet.