effusion

Effusion is the process by which a liquid escapes or accumulates in a space where it is not normally present. In physics, effusion refers to the escape of a gas through a small pore or orifice into a region of lower pressure. In medicine, effusion denotes the accumulation or leakage of fluid into a body cavity or potential space, often as a result of inflammation, infection, congestive states, or malignancy.

Gas effusion is governed by differences in pressure and temperature. At a given temperature and pressure, the rate at which a gas effuses is inversely proportional to the square root of its molar mass, a relation known as Graham's law. Lighter gases with smaller molar masses effuse more rapidly than heavier ones. Factors such as temperature, pressure, pore size, and the presence of other gases influence the rate. This phenomenon is used in experiments to separate gases and to estimate molar masses.

In medicine, effusion most commonly refers to fluid accumulating in a potential space. Pleural effusion is the buildup of fluid in the pleural cavity around the lungs, while pericardial effusion occurs around the heart. Other examples include peritoneal (abdominal) effusions and joint effusions. Causes vary and include heart failure (often producing transudates), infections or malignancies (often producing exudates), hypoalbuminemia, liver or kidney disease, and inflammatory conditions. Evaluation typically involves imaging, such as X-ray or ultrasound, and analysis of the drained fluid via thoracentesis or paracentesis. Treatment focuses on the underlying cause and may require drainage, medical therapy to reduce fluid production, or surgical interventions in some cases.