epithelium

In animals, tissue of closely packed cells that forms a surface or lines a cavity or tube. Epithelial cells line the inside surfaces of fluid or air-filled tubes and spaces within the body. Epithelium may be protective (as in the skin) or secretory (as in the cells lining the wall of the gut). Epithelial cells join each other side to side to make epithelial tissue. Epithelial cells are specialized cells. They protect cells below them and may also carry out special functions. For example, in the human lung epithelial cells line the alveoli (air spaces). These cells are very thin and form a large surface area in order to facilitate the absorption of oxygen and the loss of carbon dioxide during the process of gas exchange. They also make sure the alveoli remain moist at all times to keep them healthy.

The epithelial cells lining the airways leading to the lung from the mouth and nose are different. Here there are many are specialized ciliated cells. These cells have tiny hairs or cilia which beat backwards and forwards. They sweep fluid up and away from the lungs to the mouth. Any dirt or bacteria that have been inhaled are taken away from the lungs where they may cause damage. Smoking harms these ciliated cells.