Nearly all inflammations are accompanied by a transudation of the fluid part of the blood from the blood-vessels, and this passes in the direction of least resistance, wherever it finds room. In the case of a wound it flows from the surface, forming a serous discharge; in that of a mucous membrane it also flows from the surface, forming a catarrh; in serous cavities it accumulates, forming an inflammatory dropsy; in the tissues it passes into the serous spaces, constituting an inflammatory oedema; in the lungs it is situated in the lung alveoli, where also it produces oedema. The exuded fluid will in many cases find its way into the lymphatics, and experiment has proved that the current in the lymphatics is much increased in inflammation.
The serous exudation is somewhat different in constitution from an ordinary transudation fluid. It is much more concentrated, approaching more nearly to the liquor sanguinis, and it contains more leucocytes than the exudation in simple oedema. It is also in many cases coagulable, so that when shed it may deposit fibrine, and sometimes does so in the living body. When this occurs 'we have the fibrinous exudation. We may associate these characters of the serous exudation with the fact that in acute inflammations the tissues, including the walls of the vessels, are seriously damaged, and the latter allow more readily of the escape of the fluid of the blood.