The drainage tube is a medical device used to remove excess fluid or air from a body cavity. It is a hollow, flexible tube that is usually inserted through a small incision or puncture in the skin. 

Drainage tubes can be classified based on their size, shape, and material. They are commonly made of silicone, rubber, or plastic and can range in diameter from a few millimeters to several centimeters. 

The use of drainage tubes is common in various medical procedures, including surgery, trauma care, and critical care medicine. They are often used to drain excess fluid or blood from a surgical site, to relieve pressure caused by the buildup of fluid in the chest or abdomen, or to drain an abscess. 

Overall, drainage tubes are a valuable tool in the field of medicine. They provide a safe and effective way to manage excess fluid or air in the body and can help improve patient outcomes.