Epithelial tissues are composed of closely aggregated cells, which usually have little intercellular material between them. Epithelial tissues can line cavities, cover a body organ or structure, and form glands. Further functions of epithelial tissues are absorption of nutrients and secretion of substances. The cells may be joined to a single layer, for example, alveolar epithelia in lungs, which provide a little diffusion distance for the oxygen, or as a multiple layers, for example the skin epithelial.
The type of cells that compose the epithelial tissues may vary from cuboidal (for example in kidney tubules), to columnar (for example in the digestive tract), to flattened squamous cells (for example in blood vessels). Epithelial tissues can also form glands, which secret their products into ducts (exocrine glands), or into blood vessels (endocrine glands).