Mechanical valves (or check valves) permit gases and liquids to flow in only one direction. Some common types are the ball check valve and the flap valve. The flap valve consists of leather or rubber acting as a hinged flap which opens wide for flow in one direction, but flaps tight shut if the flow is in the other direction. Check valves are the main element in a variety of pumps. In biological systems we find the flap valve functioning in the pumping system of the heart. The concept of the valve (permitting flow in only one direction) is found in electronics. The vacuum tube and its solid state counterpart, the semiconductor diode, permit electrical current to flow in only one direction. In England, vacuum tubes are called "valves".
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