In short
Epithelial tissue is the animal tissue that covers the outside of the body and lines the inside of organs and cavities. Its cells are packed tightly with very little material between them.
It protects, absorbs, secretes and filters. It is named by its shape and layers. Squamous cells are flat, as in the lining of blood vessels and the air sacs of the lungs. Cuboidal cells are cube-shaped, as in the kidney tubules. Columnar cells are tall, as in the lining of the intestine. A simple epithelium has one layer, and a stratified one has many, as in the skin.
The thin, flat squamous epithelium in the air sacs of the lungs is only one cell thick, so gases can pass through it quickly.
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