An exudate is any fluid that filters from the circulatory system into lesions or areas of inflammation. It can apply to plants as well as animals. Its composition varies but generally includes water and the dissolved solutes of the main circulatory fluid such as sap or blood. In the case of blood it will contain some or all plasma proteins, white blood cells, platelets, and in the case of local vascular damage: red blood cells. In plants, it can be a healing and defensive response to repel insect attack, or it can be an offensive habit to repel other incompatible or competitive plants. Organisms that feed on exudate are known as exudativores; for example, the Vampire Bat exhibits hematophagy, and the Pygmy marmoset is an obligate gummivore (primarily eats tree gum).
In humans, exudate can be a pus-like or clear fluid. When an injury occurs, leaving skin exposed, it leaks out of the blood vessels and into nearby tissues. The fluid is composed of serum, fibrin, and white blood cells. Exudate may ooze from cuts or from areas of infection or inflammation.
- Purulent or suppurative exudate consists of plasma with both active and dead neutrophils, fibrinogen, and necrotic parenchymal cells. This kind of exudate is consistent with more severe infections, and is commonly referred to as pus.
- Fibrinous exudate is composed mainly of fibrinogen and fibrin. It is characteristic of rheumatic carditis, but is seen in all severe injuries such as strep throat and bacterial pneumonia. Fibrinous inflammation is often difficult to resolve due to blood vessels growing into the exudate and filling space that was occupied by fibrin. Often, large amounts of antibiotics are necessary for resolution.
- Catarhal exudate is seen in the nose and throat and is characterized by a high content of mucus.
- Serous exudate (sometimes classified as serous transudate) is usually seen in mild inflammation, with relatively low protein. Its consistency resembles that of serum, and can usually be seen in certain disease states like tuberculosis. (See below for difference between transudate and exudate)
- Malignant (or cancerous) pleural effusion is effusion where cancer cells are present. It is usually classified as exudate.
Plant exudates 
Plant roots exude a variety of molecules into the rhizosphere, including acids, sugars, polysaccharides and ectoenzymes; this can account for 40% of root carbon. Exudation of these compounds has various benefits to the plant and to the microorganisms of the rhizosphere.
Exudates vs. transudates 
|Transudate vs. exudate|
|Main causes||Increased hydrostatic
|Specific gravity||< 1.012||> 1.020|
|Protein content||< 25 g/L||> 29 g/L|
|< 0.5||> 0.5|
with blood albumin
|> 1.2 g/dL||< 1.2 g/dL|
upper limit for serum
|< 0.6 or < ⅔||> 0.6 or > ⅔|
|Cholesterol content||< 45 mg/dL||> 45 mg/dL|
There is an important distinction between transudates and exudates. Transudates are caused by disturbances of hydrostatic or colloid osmotic pressure, not by inflammation. They have a low protein content in comparison to exudates. Medical distinction between transudates and exudates is through the measurement of the specific gravity of extracted fluid. Specific gravity is used to measure the protein content of the fluid. The higher the specific gravity, the greater the likelihood of capillary permeability changes in relation to body cavities. For example, the specific gravity of the transudate is usually less than 1.012 and a protein content of less than 2 gm/100mL (2 gm%). Rivalta test may be used to differentiate an exudate from a transudate. It is not clear if there is a distinction in the difference of transudates and exudates in plants.
See also 
- Ford, Susan M.; Porter, Leila M.; Davis, Lesa C., ed. (2009). The Smallest Anthropoids: The Marmoset/Callimico Radiation. Springer US. pp. 381–394. ISBN 978-1-4419-0293-1. Retrieved 6 October 2012. "The Functional Significance of Jaw-Muscle Fiber Architecture in Tree-Gouging Marmosets"
- "Exudate". MedlinePlus.
- ""Exuded" Merriam-Webster Online Dictionary". Meriam Webster. 2008. Retrieved 2008-07-04.
- Robert K. Barnhart, ed. (1988). Chambers Dictionary of Etymology. New York: Chambers Harrap Publishers. p. 363. ISBN 0-550-14230-4.
- Robbins Basic Pathology 7th ed
- About.com > Malignant Pleural Effusion By Lynne Eldridge MD. Updated March 27, 2010
- Power, Michael L. (2010). In Anne M. Burrows, Leanne T. Nash. The Evolution of Exudativory in Primates / Nutritional and Digestive Challenges to Being a Gum-feeding Primate. Springer. p. 28. Retrieved 2 October 2012.
- Marschner, Horst (1995). Mineral Nutrition of Higher Plants. ISBN 0124735436.
- Walker, T. S.; Bais, H. P.; Grotewold, E.; Vivanco, J. M. (2003). "Root Exudation and Rhizosphere Biology". Plant Physiology 132 (1): 44–51. doi:10.1104/pp.102.019661. PMC 1540314. PMID 12746510.
- The University of Utah • Spencer S. Eccles Health Sciences Library > WebPath images > "Inflammation".
- Heffner J, Brown L, Barbieri C (1997). "Diagnostic value of tests that discriminate between exudative and transudative pleural effusions. Primary Study Investigators". Chest 111 (4): 970–80. doi:10.1378/chest.111.4.970. PMID 9106577.
- Light R, Macgregor M, Luchsinger P, Ball W (1972). "Pleural effusions: the diagnostic separation of transudates and exudates". Ann Intern Med 77 (4): 507–13. PMID 4642731.
- Roth BJ, O'Meara TF, Gragun WH (1990). "The serum-effusion albumin gradient in the evaluation of pleural effusions". Chest 98 (3): 546–9. doi:10.1378/chest.98.3.546. PMID 2152757.