What does the epithelium do?

The epithelium performs several functions that are vital to life. There are eight types of epithelial tissue. The exact function of each depends on the type of arrangement of cells in the tissue as well as its location in the body. 

• Protection – The epithelium provides the body’s first line of defense against pathogens, chemical substances, physical trauma, and excessive fluid loss. For example, the skin, which is composed of epithelial tissue, protects the blood vessels, internal organs, muscle and other tissues deeper in the body. The epithelium that lines the intestines protects the body from intestinal bacteria.
• Absorption – The epithelial lining of internal organs such as the lungs and liver allow the absorption of select substances. For example, the epithelial lining of the small intestines absorbs nutrients from the digestive tract.
• Secretion - Glandular epithelium or the epithelial tissue in the glands, secretes enzymes, hormones, saliva, sweat, and mucus, which regulate various bodily functions. 
• Exchange of substances – The epithelium regulates the exchange of substances between the body and the external environment as well as the internal exchange between the underlying tissues and the body cavity. 
• Filtration - The epithelium of the respiratory tract and the kidneys play vital roles in filtration. The respiratory tract epithelial tissues filter out unwanted particles so you only breathe in clean air. The epithelial tissue in the kidneys filters the blood.
• Diffusion - Membranes formed by simple squamous epithelial cells allow the passive movement of substances from regions of higher concentrations to regions of lower concentration. Diffusion acts together with several other functions including absorption, filtration, and secretion. 
• Sensory reception – Epithelial tissue in the eyes, nose, taste buds and ears have sensory nerve endings that receive external sensory stimuli, which is then translated into neural signals that are transmitted to the brain for processing. 
• Excretion - The epithelial tissues in the sweat glands are responsible for secreting sweat while those in the kidneys are responsible for excreting waste.