Editing Extracellular fluid (section)

==Interaction between the blood plasma, interstitial fluid and lymph==
{{Further |Starling equation |Microcirculation#Capillary exchange}}
[[File:Capillary microcirculation.svg|thumb|Formation of interstitial fluid from blood]]
[[File:Illu lymph capillary.png|thumb |Diagram showing the formation of lymph from interstitial fluid (labeled here as "tissue fluid"). The tissue fluid is entering the blind ends of [[lymph capillaries]] (shown as deep green arrows).]]
The arterial blood plasma, interstitial fluid and lymph interact at the level of the blood [[capillary|capillaries]]. The capillaries are [[Vascular permeability|permeable]] and water can move freely in and out. At the [[arteriole|arteriolar end]] of the capillary the blood pressure is greater than the [[Hydrostatics#Medicine|hydrostatic pressure]] in the tissues.<ref name="Guyton">{{cite book|last1=Hall|first1=John | name-list-style = vanc |title=Guyton and Hall textbook of medical physiology|date=2011|publisher=Saunders/Elsevier|location=Philadelphia|isbn=978-1-4160-4574-8|pages=177–181|edition=12th}}</ref><ref name=tortora /> Water will therefore seep out of the capillary into the interstitial fluid. The pores through which this water moves are large enough to allow all the smaller molecules (up to the size of small proteins such as [[insulin]]) to move freely through the capillary wall as well. This means that their concentrations across the capillary wall equalize, and therefore have no osmotic effect (because the osmotic pressure caused by these small molecules and ions – called the [[Crystalloid solution|crystalloid osmotic pressure]] to distinguish it from the osmotic effect of the larger molecules that cannot move across the capillary membrane – is the same on both sides of capillary wall).<ref name="Guyton" /><ref name=tortora />

The movement of water out of the capillary at the arteriolar end causes the concentration of the substances that cannot cross the capillary wall to increase as the blood moves to the [[Venule|venular]] end of the capillary. The most important substances that are confined to the capillary tube are [[Serum albumin|plasma albumin]], the [[Globulin|plasma globulins]] and [[fibrinogen]]. They, and particularly the plasma albumin, because of its molecular abundance in the plasma, are responsible for the so-called [[Oncotic pressure|"oncotic" or "colloid" osmotic pressure]] which draws water back into the capillary, especially at the venular end.<ref name="Guyton" />

The net effect of all of these processes is that water moves out of and back into the capillary, while the crystalloid substances in the capillary and interstitial fluids equilibrate. Since the capillary fluid is constantly and rapidly renewed by the flow of the blood, its composition dominates the equilibrium concentration that is achieved in the capillary bed. This ensures that the [[Milieu intérieur|watery environment]] of the body's cells is always close to their ideal environment (set by the body's [[Homeostasis|homeostats]]).

A small proportion of the solution that leaks out of the capillaries is not drawn back into the capillary by the colloid osmotic forces. This amounts to between 2–4 liters per day for the body as a whole. This water is collected by the [[Lymphatic vessel|lymphatic system]] and is ultimately discharged into the left [[subclavian vein]], where it mixes with the venous blood coming from the left arm, on its way to the heart.<ref name=tortora /> The [[lymph]] flows through [[Lymph capillary|lymph capillaries]] to [[lymph node]]s where bacteria and tissue debris are removed from the lymph, while various types of [[white blood cell]]s (mainly [[lymphocyte]]s) are added to the fluid. In addition the lymph which drains the small intestine contains fat droplets called [[chylomicron]]s after the ingestion of a fatty meal.<ref name=stryer /> This lymph is called [[chyle]] which has a milky appearance, and imparts the name [[lacteal]]s (referring to the milky appearance of their contents) to the lymph vessels of the small intestine.<ref name=grays>{{cite book |last1=Williams |first1=Peter L |last2=Warwick |first2=Roger |last3=Dyson|first3=Mary |last4=Bannister |first4=Lawrence H. | name-list-style = vanc |title=Gray's Anatomy| pages=821 |location=Edinburgh|publisher=Churchill Livingstone | edition=37th |date=1989|isbn= 0443-041776 }}</ref>

Extracellular fluid may be mechanically guided in this circulation by the vesicles between other structures.  Collectively this forms the [[interstitium]], which may be considered a newly identified biological structure in the body.<ref>{{cite news|last1=Rettner|first1=Rachel| name-list-style = vanc |title=Meet Your Interstitium, a Newfound 'Organ'|url=https://www.scientificamerican.com/article/meet-your-interstitium-a-newfound-organ/|access-date=28 March 2018|work=[[Scientific American]]|date=27 March 2018}}</ref> However, there is some debate over whether the interstitium is an organ.<ref>{{Cite web|url=https://www.the-scientist.com/?articles.view/articleNo/52168/title/Is-the-Interstitium-Really-a-New-Organ-/|title=Is the Interstitium Really a New Organ? |website=The Scientist }}</ref>