Answer: a. The lining of a blood vessel is simple squamous epithelium. This lining is called endothelium. Epithelium lines body cavities and surfaces. Simple squamous epithelium is simple because it is one cell thick. Squamous refers to the fact that the cells are flat.3.pithelium lines body cavities and surfaces. Pseudostratified squamous epithelium is pseudostratified because it is only one cell layer thick, yet it appears to be stratified. In reality, every cell touches the basement membrane.