Editing Edema (section)

== Mechanism ==
Six factors can contribute to the formation of edema:<ref>{{cite book |last1=Scallan |first1=Joshua |last2=Huxley |first2=Virginia H. |last3=Korthuis |first3=Ronald J. |title=Pathophysiology of Edema Formation |date=2010 |publisher=Morgan & Claypool Life Sciences |url=https://www.ncbi.nlm.nih.gov/books/NBK53445/ |access-date=11 July 2022 |language=en}}</ref>
# increased [[hydrostatic pressure]];
# reduced colloidal or [[oncotic pressure]] within blood vessels;
# increased tissue colloidal or  oncotic pressure;
# increased blood vessel wall permeability (such as [[inflammation]]);
# obstruction of fluid clearance in the [[lymphatic system]];
# changes in the water-retaining properties of the tissues themselves. Raised hydrostatic pressure often reflects retention of water and sodium by the kidneys.<ref>{{cite book| author=Kumar|author2=Abbas|author3=Fausto |title=Pathologic Basis of Disease |edition=7th |publisher=Elsevier Saunders |year=1999 |pages=122 |isbn= 0-7216-0187-1}}</ref>
Generation of interstitial fluid is regulated by the forces of the [[Starling equation]].<ref>{{cite book | last1= Boron | first1= W. F. | last2= Boulpaep | first2= E. L. | year = 2012| title = Medical Physiology: A Cellular and Molecular Approach| chapter = 2e| publisher =Saunders/Elsevier| place = Philadelphia| pages =| isbn=}}</ref> Hydrostatic pressure within blood vessels tends to cause water to filter out into the tissue. This leads to a difference in protein concentration between blood plasma and tissue. As a result, the colloidal or [[oncotic pressure]] of the higher level of protein in the plasma tends to draw water back into the blood vessels from the tissue. Starling's equation states that the rate of leakage of fluid is determined by the difference between the two forces and also by the permeability of the vessel wall to water, which determines the rate of flow for a given force imbalance. Most water leakage occurs in capillaries or post capillary [[venule]]s, which have a [[semi-permeable membrane]] wall that allows water to pass more freely than protein. (The protein is said to be reflected and the efficiency of reflection is given by a reflection constant of up to 1.) If the gaps between the cells of the vessel wall open up then permeability to water is increased first, but as the gaps increase in size permeability to protein also increases with a fall in reflection coefficient.<ref>{{cite journal |last1=Dvorak |first1=Harold F |title=Vascular permeability to plasma, plasma proteins, and cells: an update |journal=Current Opinion in Hematology |date=April 2010 |volume=17 |issue=3 |pages=225–229 |doi=10.1097/MOH.0b013e3283386638 |pmid=20375889 |pmc=2878124 }}</ref>

Changes in the variables in Starling's equation can contribute to the formation of edemas either by an increase in hydrostatic pressure within the blood vessel, a decrease in the oncotic pressure within the blood vessel or an increase in vessel wall permeability. The latter has two effects. It allows water to flow more freely and it reduces the colloidal or oncotic pressure difference by allowing protein to leave the vessel more easily.{{citation needed|date=April 2022}}

Another set of vessels known as the [[lymphatic system]] acts like an "overflow" and can return much excess fluid to the [[bloodstream]]. But even the [[lymphatic system]] can be overwhelmed, and if there is simply too much fluid, or if the [[lymphatic system]] is congested, then the fluid will remain in the tissues, causing swellings in [[human leg|leg]]s, [[ankle]]s, feet, [[abdomen]] or any other part of the body.<ref>{{cite book |title=Essentials of Rubin's Pathology |first=Emanuel |last=Rubin |year=2008 |edition=5th |location=Philadelphia |publisher=Lippincott Williams & Wilkins |page=124 |isbn=978-0-78177-324-9}}</ref>