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Transudation consists in the passage of a liquid, the so-called transudate, through a membrane; typical is the example of the liquid component of the blood through the capillary wall.

By definition, the transudate is particularly poor in proteins and cellular elements; in this sense it differs from "exudate, which recognizes an" important solid component formed by plasma proteins, blood cells and substances deriving from the destruction or metabolic activity of the injured tissue.

The exudate is therefore consequent to episodes of increased permeability of the capillary membrane, which is allowed to pass through by solutes normally retained within the bloodstream. Typical is the example of inflammatory phenomena (trauma, injuries of various kinds, infections, etc.). In the formation of the transudate, on the contrary, the capillary wall maintains its normal filtering capacity (there is no phlogistic component); on the other hand, the increased capillary and venous blood pressure plays a leading role. Transudation is also favored by the low level of plasma proteins, as occurs in hypoalbuminemia; according to the laws of osmosis (oncotic or colloid-osmotic pressure), in fact, the more the blood is rich in proteins and the more water it draws into it from the interstitial fluid, and vice versa. To cite an example, the abnormal accumulation of fluid in the abdominal cavity (ascites) is typical of both severe calorie-protein malnutrition (Kwashiorkor) and all conditions associated with portal hypertension, such as cirrhosis.

In women, the so-called "vaginal secretions" which lubricate the canal during sexual arousal, are formed by transudate, since this tract - unlike the external opening and the cervix - is devoid of glands.