duct

A duct is a tubular structure specialized for transporting secreted or excreted substances produced by glands — such as bile, saliva, enzymes, or hormones. Unlike blood vessels (which carry blood) or lymphatics (which carry lymph), ducts convey the products of specific glandular tissues to their destination, whether that is another organ or a body surface. Ducts are lined by epithelium whose cell type and properties vary with the substance being transported.

Major ducts include: the common bile duct (carries bile from the liver and gallbladder to the duodenum), the pancreatic duct (carries pancreatic enzymes to the duodenum), the parotid duct or Stensen's duct (carries saliva from the parotid gland to the mouth), the thoracic duct (largest lymphatic vessel, carries lymph to the venous circulation), and the vas deferens (carries sperm from the epididymis to the ejaculatory duct).

Duct obstruction prevents the normal outflow of secretions, causing them to back up and potentially damage the gland that produces them. Bile duct obstruction (by gallstones or tumor) causes jaundice and cholestasis. Pancreatic duct obstruction leads to pancreatitis, where digestive enzymes begin digesting the pancreas itself. Salivary duct obstruction (by calculi or strictures) causes painful swelling of the affected gland, especially during eating.