Most edema is an accumulation of lymph fluid, which is usually just the non-cellular component of blood. It's mostly water, with electrolytes and other organic substances in solution. Unlike the circulatory system, which is driven by the pumping heart, the lymphatic system does not have a central pump. Lymph ducts have one-way gates in them so it all eventually returns back to your trunk, but it's easy for lymphatic fluid to accumulate in the extremities due to simple forces like gravity, compression from clothing, etc.
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