At the right is a probe patent foramen ovale in an adult. A metal probe lifts the septum secundum and reveals the opening. Normally, the left atrial pressure keeps the foramen closed, but if right atrial pressures rise with pulmonary hypertension (as with pulmonary embolus), the foramen may open and even allow a thrombus to go from right to left. This is a "paradoxical embolus", rare (seen on the left here), and so called because a thromboembolus arising from the venous circulation can end in the systemic circulation.

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