When inflammation is present in the body, histamine is released. Histamine causes the gaps between the cells of the capillary walls to widen, making them leakier. The aim is to make it easier for infection-fighting white blood cells to quickly get to the site of an inflammation. However, if the inflammation persists for a long time, water retention can become a problem
The lymphatic system drains fluid from tissues (called lymph) and empties it back into the bloodstream. However, if too much fluid is released in the first place, the lymphatic system can be overwhelmed - it is unable to return fluid fast enough, and it accumulates around the tissues.
Sometimes, if the lymphatic system is congested, the rate at which fluid is returned to the bloodstream may change. This means that fluid might remain in the tissues, causing swelling in various parts of the body, including the abdomen, ankles, legs, and feet.