Epithelial cell layer covers the organs and cavities inside the body and also covers the whole body with skin. It provides a large surface area provided by microvilli. In lung alveoli, gas exchange takes place. In blood vessels, exchange of gases, minerals takes place. In kidney tubules, various salts are exchanged. For this purpose, the outer lining made by epithelial layer should be selectively permeable as per the requirement. In all these mechanisms, the composition of epithelial layer changes as per requirement as an exchange of different material demands different cellular composition. So, exchange of materials between body and external environment is regulated by epithelial layer.