Epithelial tissue or epithelium forms the outer covering of the skin and also lines the body cavity. It forms the lining of respiratory, digestive, reproductive and excretory tracts. They perform various functions such as absorption, protection, sensation and secretion. Epithelial tissue is formed from a tightly fitted continuous layer of cells. One surface of the epithelial tissue is exposed to either the external environment or the body fluid. The other surface is attached to tissue by a membrane, which consists of fibres and polysaccharides secreted by epithelial cells. There is little intercellular material present between cells. There are specialised junctions present between the cells of the epithelium, that link individual cells. Protection: As it covers the entire body surface, it is the first line of defence against any kind of mechanical injury, chemical exposure, excessive fluid loss and infections. Ciliary projections present in the nose or upper respiratory tract, trap the dust particles and prevent it from entering the body The epithelial lining of the digestive tract absorbs water and nutrients. Epithelial tissue regulates the exchange of substances between body and external environment as well as the internal exchange between different parts of the body. Everything that enters the body or enters the bloodstream by absorption has to cross the epithelial barrier. Sensory receptors are present in the epithelial tissue of the nose, eyes and ears, taste bud, etc. that help in transmitting signals from the external stimuli to the brain. Various glands made up of epithelial cells secrete hormones, enzymes, saliva, mucus, sweat, etc. Source: https://byjus.com/neet/epithelial-tissue/