In order for a physical barrier to be effective in preventing infection, the barrier must be able to prevent a pathogen from entering the body, trap the pathogen when it first enters the body or flush it out of the body. Physical barriers of the body include the skin, mucous membranes, earwax, tears, mucus and stomach acid. The normal flow of urine also washes out microorganisms that try to enter the urinary tract. When the bladder empties, it flushes out any bacteria that may be attempting to reach it