Simple squamous epithelia are tissues formed from one layer of squamous cells that line surfaces. Squamous cells are large, thin, and flat and contain a rounded nucleus. Like other epithelial cells, they have polarity and contain a distinct apical surface with specialized membrane proteins. These epithelia are common where absorption or transport of materials is important. They also play a role in diffusion, osmosis and filtration. This makes them important in the kidney, in the alveoli of lungs and in the walls of capillaries.