Atmospheric pressure is applied to the entire body as so does not influence the flow of fluid from one part to another
The gradients of pressure that create the flow of blood along arteries or veins to or from one part of the body to another are not altered. This is not the same as for instance a compressive bandage applies to an arm or leg. A compressive dressing changes the gradient of pressure when it is applied to only one part of the body.
Fluids including edema fluid is not compressible but gases are compressible so gas in a confined space with expand with negative pressure and contract with positive pressure. The middle ear contains air under normal circumstances. This air will expand during decompression as when passengers experience on an airliner as it descends in altitude. Normally the Eustachian tube allows pressure to be equalized between the air in the middle ear and the atmosphere Normally the pressure across the thin and sensitive tympanic membrane. If there is dysfunction of the Eustachian tube then there may be a build up of pressure pushing against this membrane. This may be felt as a discomfort. If by opening the mouth widely to Eustachian tube opens the ear may pop, pressure may equalize and the discomfort usually improves. Hearing may also improve as the tympanic membrane will move more freely without the pressure pushing against one side of it.
The bowels often contain air. Generally the gas will be relieved with any expansion without causing injury or discomfort. An exception may occur in a closed loop bowel obstruction. This is when there is an obstruction at both ends of a loop of bowel. If there is air in this loop then decompression of the atmosphere could theoretically cause this air in this closed space to expand and distend the loop slightly. This would be a strange and almost unimaginable circumstance. Suffice it to say that persons suffering a bowel obstruction with a closed loop of bowel don’t usually board an airliner.
Dependent edema results from a body part being below or dependent to the rest of the body. In this case fluid pressure not air pressure creates a gradient resulting from gravity. Feet and ankles can of course develop edema when a person is sitting.
Air pressure in the cabin of a pressurized aircraft is not usually the same or near sea level pressure. Air cabin pressure is typically about the same as about seven to eight thousand feet when cruising at altitude.
Traveling on an airliner, the risk if any to the typical reduced air pressure is the mild reduction in the ability of oxygen at reduced pressure to function as well as at sea level pressure. There is no effect on the edema of any tissue and there is only a trivial nuisance of air expanding in the middle ear causing discomfort.