What are the functions of epithelial tissue?

Epithelial tissue has various important functions throughout the body. These functions include: secretion, excretion, adsorption, protection, diffusion, transportation, sensory reception, and filtration. Epithelial cells have the capability to secrete multiple macromolecules. The glandular epithelium is one common example, as endocrine glands release hormones which modulate different processes such as blood sugar levels (insulin) and the cardiac cycle (noradrenaline). 

Epithelial tissue also has a selective barrier for protecting the organs from external stresses such as tearing and infections. This is why the epithelium is avascular, as cuts would result in the tearing and subsequent bleeding of the vessel. Simple squamous epithelial tissues generate a membrane which permits the selective diffusion of substances to pass through. Stratified squamous keratinized epithelium of the skin has a high cell renewal rate following injury. Epithelial cells also have absorption function, via their surface epithelia with apical microvilli. These microvilli increase the absorptive surface area. As an example, columnar epithelium in the small intestine absorbs nutrients from the digestive tract and delivers digested substances into the bloodstream.  

Epithelial cells are able to transport substances out of their cells via channels and pumps on their apical and basal surfaces. For example, columnar cells of the ileum deliver iron from the intestine to capillaries. Another example is the cuboidal cells of the renal tubule release H+ from the body into the urine. The epithelium is also able to receive sensory information and subsequently convert it into neural signals. For example, pseudostratified columnar epithelium of the olfactory nasal cavity has receptor cells with apical cilia used for detecting chemical signals. The signal is transported to the olfactory nerve which then sends the information to the CNS for interpretation by the brain. Epithelial tissue is also able to filter out substances. For example, epithelium of the respiratory tract filters out dirt and other material to allow for clean air to be inhaled in.