The epithelium is a basic kind of tissue which lines the outer (such as the skin) and inner (such as blood vessels) surfaces of organs. There are different kinds of epithelial tissues. The squamous epithelium is composed of wide and flat cells which are found in locations such as the lungs’ alveoli, blood vessels, esophagus, and lining of the mouth. It secretes lubrication and allows substances to diffuse and filtrate. As its name suggests, the cuboidal epithelium consists of cube-shaped cells and the areas where it can be found include ducts of small glands and kidney tubules. It is mainly important for secretion, absorption, and protection. The columnar epithelium has column-shaped cells and they are found in locations such as the uterus, bronchi, digestive tract, and bladder. It is also vital for absorption and secretion of enzymes and mucous.