The process by which fluid escapes. The term also describes an abnormal collection of fluid, such as blood, pus, or plasma, in the tissues or a body cavity. An effusion can form as a result of inflammation or changes in pressure within blood vessels, or it can be due to changes in blood constituents, as in nephrotic syndrome. Effusion commonly occurs around the lung ( pleural effusion) or heart ( pericardial effusion) or within joints, causing swelling.


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