Edema is a pathological phenomenon of excess fluid to gather in the interstitial space. Mild fluid retention cannot cause edema, you can see the naked eye edema until the body fluid storage capacity is more than 4–5 kg. Edema is divided into systematic and non-systematic. Systemic edema means the liquid distributed diffusely in the body tissue space, shows swell of many parts on body and the skin sunken after a long time press, what also known as depression edema. Pleural effusion, pericardial effusion and ascites are respectively called when the fluid accumulates in pleural, pericardial, peritoneal cavity. In general, systematic edema often means that the body has a serious disease, but local cause or systematic diseases can also causes non-systematic edema.