In anatomy and physiology, a duct is a circumscribed channel leading from an exocrine gland or organ.

Epithelium or epithelial tissue is a thin, continuous, protective layer of compactly packed cells with little extracellular matrix. Epithelial tissues line the outer surfaces of organs and blood vessels throughout the body, as well as the inner surfaces of cavities in many internal organs. An example is the epidermis, the outermost layer of the skin. Epithelial tissue is one of the four basic types of animal tissue, along with connective tissue, muscle tissue and nervous tissue. These tissues also lack blood or lymph supply. The tissue is supplied by nerves.