Epithelial tissues are large sheets of cells covering all of the surfaces of the body. These surfaces can be internal or external, for example, skin, airways, the digestive tract, the urinary system, and the reproductive system. Hollow organs and body cavities that do not connect to the body's exterior, including blood vessels and serous membranes, are lined by epithelial tissue known as the endothelium.

Epithelial tissues provide the body's first line of protection from physical, chemical, and biological wear and tear. They act as the gatekeepers of the body and allow the selective transfer of materials across a physical barrier. This means that all substances that enter the body must cross an epithelium. Many epithelial cells secrete mucus or other chemical compounds with specific functions onto their apical surfaces. For example, the intestinal epithelium releases digestive enzymes to help digest food, while the respiratory tract lining secretes mucus to trap incoming microorganisms and particles.

Additionally, the epithelium also forms much of the glandular tissue of the body. Glandular tissue consists of single or multiple secretory cells that yield a variety of secretions. These secretions are discharged on the epithelial surface or released into the bloodstream to reach target cells.

Epithelial tissues also have a large supply of sensory nerves that help to generate an immediate response to stimulation.

This text is adapted from Openstax, Anatomy and Physiology 2e, Section 4.2: Epithelial Tissues