The food you eat takes an incredible journey through your body, from top (your mouth) to bottom (your anus). Along the way the beneficial parts of your food are absorbed, giving you energy and nutrients. Here's a step-by-step account of the digestive system's workings. In general, epithelial tissues are classified by the number of their layers and by the shape and function of the cells. The three principal shapes associated with epithelial cells are squamous, cuboidal, and columnar.