There are four primary tissues as follows: epithelial, connective, muscular, and nervous.
a. Epithelial (Figure 2-5). This tissue covers the outer surface of the body and forms the lining of the intestinal and respiratory systems. A special form called endothelium lines the heart and blood vessels. As serous membranes, it lines the cavities of the abdomen, the chest, and the heart, and covers the organs that lie in these cavities. Epithelial tissue forms the glands and parts of the sense organs. According to its location, this tissue has different functions. As the skin, it protects underlying structures; in the small intestine, it absorbs; in the lungs, it is a highly permeable membrane; in glands, it secretes; and in the kidneys and liver, it both secretes and excretes. There are three types of epithelial tissue based on the shape of the cells. These are squamous (flat), cuboidal, and columnar. These cells are further
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