Here is an example of fluid collection into a body cavity, or an effusion. This is a right pleural effusion (in a baby). Note the clear, pale yellow appearance of the fluid. This is a serous effusion. Extravascular fluid collections can be classified as follows:
Effusions into body cavities can be further described as follows:
Exudate: extravascular fluid collection that is rich in protein and/or cells. Fluid appears grossly cloudy.
Transudate: extravascular fluid collection that is basically an ultrafiltrate of plasma with little protein and few or no cells. Fluid appears grossly clear.
Serous: a transudate with mainly edema fluid and few cells.
Serosanguinous: an effusion with red blood cells.
Fibrinous (serofibrinous): fibrin strands are derived from a protein-rich exudate.
Purulent: numerous PMN's are present. Also called "empyema" in the pleural space.