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:
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.
Effusions into body cavities can be further described as follows:
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.