A tissue which functions in protection, absorption, and secretion. When serving a protective function, epithelial tissue is found in sheets, covering a surface such as the skin. In the absorptive function, the epithelial tissues form specialized surfaces, as in the tiny air sacs of the lung tissue, where oxygen is absorbed into the blood. In the functions of secretion, there are two specialized epithelial tissues - mucous membrane and glandular epithelium.