After surgery, a drain is usually placed, depending on the site of surgery, in the abdominal cavity, thoracic cavity, subcutaneous, ventricles, etc. The principle is usually related to the siphon principle or negative pressure suction principle. After surgery, indwelling drains are often needed to direct fluids such as exudate, pus, blood, and saline used for irrigation during surgery that accumulate between body tissues or in body cavities to the outside of the body, to prevent postoperative infections, and to promote wound healing. Fluid in the cavity at a higher position in the body can flow into the drainage bag at a lower position through the drainage tube, i.e., the siphon principle. Normal thoracic and abdominal cavities are in a state of negative pressure, through a negative pressure value is greater than the negative pressure of the thoracic and abdominal cavities of the container (such as negative pressure ball, etc.), can be sucked to the cavity of the liquid, gas, the negative pressure value of the larger container, which is the negative pressure suction principle. It is important to note that placing a drain may cause pain. If the pain originates from within the body cavity, contact your doctor for examination.