Characteristics of Epithelial Tissues Practice Test

Gas exchange needs a very thin barrier between air and blood, so the alveolar sacs are lined by a single layer of flattened cells. This simple squamous epithelium is made up of type I pneumocytes, which are specialized to cover most of the alveolar surface and form the respiratory membrane with nearby capillaries, allowing O2 and CO2 to diffuse rapidly. A thicker, multi-layered lining like stratified squamous would impede diffusion and isn’t suited for gas exchange. Transitional epithelium is used in stretchier organs like the bladder, and simple cuboidal epithelium lines many glands and kidney tubules, not the delicate walls of alveoli. So the lining of the alveolar sacs is simple squamous epithelium.