I know that alveoli are air sacs in the lungs where gas exchange occurs with the blood.

1. I know that alveoli are air sacs in the lungs where gas exchange occurs with the blood. The tissue that makes up alveoli is simple squamous epithelium, which is different than the respiratory tract which is mostly pseudostratified columnar ciliated epithelium. why there is a difference in tissue types?

2. Is the skin is a tissue or an organ?

3. What type of epithelial tissue that is more important than the others?