All exposed surfaces of the body, including the skin and the alimentary, respiratory, and genitals tracts, are lined with epithelial cells (figure 15.2). These cells are tightly packed together and rest on a thin layer of fibrous material, the basement membrane. In addition to the physical protection provided by this physical barrier against the outside world, the body's surfaces are bathed with a variety of antimicrobial substances that either kill or inhibit many microbes (figure 15.3). Certain microbes, however, are highly adapted to these conditions and actually grow, providing other types of additional protection.
In this chapter, we will describe the general physical and chemical aspects of the anatomical barriers. We will also discuss the protective contributions of the normal flora, those microbes that routinely inhabit the body surfaces. Various other first-line defense mechanisms are discussed more fully in the chapters dealing with each body system. ■ normal flora, p. 461
The skin is the most visible barrier, covering the majority of surfaces that are in obvious contact with the environment. Mucous membranes line the alimentary tract, respiratory tract, and genitourinary tract. These surfaces are often considered to be "inside" the body, but actually they are in direct contact with the external environment. For example, the alimentary tract, which begins at the mouth and ends at the anus, is simply a hollow tube that runs through the body, providing the opportunity for intestinal cells to absorb nutrients from food that passes (see figure 24.1); the respiratory tract is a cavity that allows oxygen and carbon dioxide gases to be exchanged (see figure 23.1).
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