Epithelial functions include protection absorp tion

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Epithelial functions include protection, absorp- tion, filtration, and secretion . For example, the epithelium of the skin protects against bacterial and chemical damage and that lining the respira- tory tract has cilia, which sweep dust and other de- bris away from the lungs. Epithelium specialized to absorb substances lines some digestive system or- gans such as the stomach and small intestine, which absorb food into the body. In the kidneys, epithelium both absorbs and filters. Secretion is a specialty of the glands, which produce such sub- stances as perspiration, oil, digestive enzymes, and mucus. Special Characteristics of Epithelium Epithelium generally has the characteristics listed below: Except for glandular epithelium (described on p. 90), epithelial cells fit closely together to form continuous sheets. Neighboring cells are bound together at many points by cell junctions, includ- ing desmosomes and tight junctions. The membranes always have one free (unat- tached) surface or edge. This so-called apical surface is exposed to the body’s exterior or to the cavity of an internal organ. The exposed surfaces of some epithelia are slick and smooth, but others exhibit cell surface modifi- cations, such as microvilli or cilia. The lower surface of an epithelium rests on a basement membrane, a structureless material secreted by the cells. Epithelial tissues have no blood supply of their own (that is, they are avascular ) and depend on diffusion from the capillaries in the under- lying connective tissue for food and oxygen. If well nourished, epithelial cells regenerate themselves easily. Classification of Epithelium Each epithelium is given two names. The first indi- cates the relative number of cell layers it has (Figure 3.17a). The classifications by cell arrange- ment (layers) are simple epithelium (one layer of cells) and stratified epithelium (more than one cell layer). The second describes the shape of its cells (Figure 3.17b). On this basis there are squamous (skwa mus) cells, flattened like fish scales ( squam scale), cuboidal (ku-boi dal) cells, which are cube-shaped like dice, and columnar cells, shaped like columns. The terms describing the shape and arrangement are then combined to describe the epithelium fully. Stratified epithelia are named for the cells at the free surface of the epithelial mem- brane, not those resting on the basement membrane. Simple Epithelia The simple epithelia are most concerned with ab- sorption, secretion, and filtration. Because simple epithelia are usually very thin, protection is not one of their specialties. Simple Squamous Epithelium Simple squamous epithelium is a single layer of thin squamous cells resting on a basement membrane. The cells fit closely together, much like floor tiles. This type of epithelium usually forms membranes where filtra- tion or exchange of substances by rapid diffusion occurs. It is in the air sacs of the lungs, where 86 Essentials of Human Anatomy and Physiology
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oxygen and carbon dioxide are exchanged (Figure 3.18a), and it forms the walls of capillaries, where
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