Edema is swelling caused by the abnormal buildup of fluid in the body. The fluid collects under the skin within the tissues that are outside of the circulatory system. The circulatory system carries blood through the body. Edema is most common in the feet and legs. It can also occur in the hands, arms, face, and abdomen. When edema occurs in the abdomen, doctors call it ascites. When it occurs around the lungs, doctors call it pleural effusion.