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the intermittent opening and closing of an orifice by a buoyant, ball-shaped mass, which acts as a valve. Some kinds of objects that may act in this manner are kidney stones, gallstones, and blood clots.
Intermittent obstruction of a passageway or opening so that the flow of fluid or air is prevented from moving in and out in equal amounts. A common example is a mucus plug in a distal airway. During inspiration the airway widens allowing gas to flow past the plug. During expiration the airway narrows and gas is trapped. This mechanism can be responsible for a pneumothorax in ventilated patients.
See also: action