occurring as sheets of cells, epithelial tissues, or epithelia (singularities within the body because epithelial cells are closely packed,often with tight junctions (see figure 6.32), they function as barier against mechanical injury, pathogens, and fluid loss. epithelia also from active interfaces with the environment. for example,the epithelium that lines the nasal passages is crucial for olfaction, the sense of smell. note how different cell shapes and arrangements correlate with distinct functions.