barrier /ber″ē·ər/ [ME, barrere] , 1. a wall or other obstacle that can restrain or block the passage of substances. Barrier methods of contraception, such as the condom or cervical diaphragm, prevent the passage of spermatozoa into the uterus. Membranes and cell walls of body tissues function as screenlike barriers to permit the movement of water or certain other molecules from one side to the other while preventing the passage of other substances. Skin is an important barrier that protects against the entry of microorganisms and the exit of body fluids. Barriers in kidney tissues adjust automatically to regulate the retention or excretion of water and other substances according to the needs of organ systems elsewhere in the body. 2. something nonphysical that obstructs or separates, such as barriers to communication or compliance. 3. (in radiography) any device that intercepts beams of x-rays. A primary barrier is one that blocks the passage of the useful x-ray beam, such as the walls and floor. A secondary barrier is one that intercepts only leakage and scattered x-ray emissions. An example is the ceiling.
