Epithelium (plural, epithelia) is the  tissue  that  covers  and  lines  both the external and internal body surfaces,  including  vessels  and small cavities.  Epithelium not  only  serves  as  a  protective  covering  or lining but is also involved in  tissue  absorption,  secretion, sensory, and other specialized functions.
It serves to protect the more complex inner structures from physical, chemical, and  pathogenic  attack,  as  well as dehydration and heat loss by its formation as an  epithelial  barrier.