Epithelial simply means, the outer layer, for these tissues to work as tubes they must be well-sealed tubes that prevent any liquid or gas escaping from between cells. These cells are designed to have ‘tight junctions’ between cells that stop this escape happening.
Epithelial tubes within organisms are essential for delivering liquids and gases around the body. They are also essential to hormone producing organs and other glands such as the pancreas and salivary glands. The tubes can vary in size however all exhibit shared key features such as a polarised epithelium that exhibits differences in protein expression and cell architecture between the two sides.