# Epithelium The **epithelium** is the general name for a type of multicellular structure composed of a single layer of cells bound tightly to each other (i.e. gut or respiratory tract linings) or multiple layers (i.e. epidermis/skin) The epithelium lines both the internal and external cavities of the body, with the internal linings often having a significantly larger surface area than that of external ones - surface area of skin is much smaller than that of intestinal tract ### Endothelium **Endothelium** is a type of epithelial tissue that lines the interior surface of blood vessels and lymphatic vessels. It plays a crucial role in: - Regulating the passage of substances between the bloodstream and surrounding tissues - Controlling blood clotting - Mediating inflammation ### Mesothelium