In normal blood flow is so slow in the lower limbs when the person is sitting or standing when it is active.
In addition, when the aircraft climbs at altitude, and although the cabin is pressurized, the air pressure while in normal dimminue it allows (among other mechanisms) to promote venous return, ie the drainage oxygen-poor blood to the heart.
These two mechanisms combined create a stagnation of blood in lower limbs, increasing venous pressure in leg veins, and under this pressure, a little liquid (water, blood plasma …) passes through the walls of veins to lodge in the adjacent tissue: a slight swelling!
feet swell when you take flight.
Swollen feet causes.
That is why the feet tend to swell when you fly, and why some air companies offer their passengers elastic stockings which seek, by compressing the calves, to compensate the two mechanisms explained above!