In medicine, a stent is an expandable wire mesh tube that is inserted into a hollow structure of the body to keep it open. Stents are used on diverse structures such as coronary arteries, other blood vessels, the common bile duct, the esophagus, the trachea or large bronchi, and ureters. Prior to use, a stent is collapsed to a small diameter; when brought into place it is expanded using an inflatable balloon and is then held in place by its own tension. Stents are usually inserted by endoscopy or other procedures less invasive than a surgical operation, which makes them suitable for patients with advanced disease for whom an operation might be too dangerous. Stents may consist of wire mesh alone (some stents are coated with substances which prevent an inflammatory response), and generally are eventually covered by epithelial tissue.