The epithelium is the largest body part, even larger than the skin! In fact, the skin is part of the epithelium. Any body part that is lined is part of the epithelium. That includes the skin but also any hollow part inside of the body. For example, the blood vessels are hollow, and they are lined with epithelium. The lining of the stomach, intestines, bladder, and other hollow body organs are also included in the epithelium. When you put together all the “linings” of the body, they are far larger than any other body part. Many types of cells make up the many different parts of the epithelium, but they are all generally referred to as epithelial cells.