Editing Oncotic pressure (section)

==Description==
Throughout the body, dissolved compounds have an osmotic pressure.  Because large [[Blood proteins|plasma proteins]] cannot easily cross through the [[capillary]] walls, their effect on the osmotic pressure of the capillary interiors will, to some extent, balance out the tendency for fluid to leak out of the capillaries. In other words, the oncotic pressure tends to pull fluid into the capillaries. In conditions where plasma proteins are reduced, e.g. from being lost in the [[urine]] ([[proteinuria]]), there will be a reduction in oncotic pressure and an increase in filtration across the capillary, resulting in excess fluid buildup in the tissues ([[edema]]).

The large majority of oncotic pressure in capillaries is generated by the presence of high quantities of [[albumin]], a protein that constitutes approximately 80% of the total oncotic pressure exerted by blood plasma on interstitial fluid {{Citation needed|date=November 2019}}.  The total oncotic pressure of an average capillary is about 28 mmHg with albumin contributing approximately 22 mmHg of this oncotic pressure, despite only representing 50% of all protein in blood plasma at 35-50 g/L.<ref name=":0">{{Citation|last1=Gounden|first1=Verena|title=Hypoalbuminemia|date=2021|url=http://www.ncbi.nlm.nih.gov/books/NBK526080/|work=StatPearls|place=Treasure Island (FL)|publisher=StatPearls Publishing|pmid=30252336|access-date=2021-12-09|last2=Vashisht|first2=Rishik|last3=Jialal|first3=Ishwarlal}}</ref><ref name=":1">{{Cite book|last1=Guyton|first1=Arthur C.|url=http://archive.org/details/textbookmedicalp00acgu|title=Textbook of medical physiology|last2=Hall|first2=John E. (John Edward)|date=2006|publisher=Philadelphia : Elsevier Saunders|others=Library Genesis|isbn=978-0-7216-0240-0}}</ref>  Because blood proteins cannot escape through capillary endothelium, oncotic pressure of capillary beds tends to draw water into the vessels. It is necessary to understand the oncotic pressure as a balance; because the blood proteins reduce interior permeability, less plasma fluid can exit the vessel.<ref name=":1" />

Oncotic pressure is represented by the symbol [[Pi (letter)|Π or π]] in the [[Starling equation]] and elsewhere. The Starling equation in particular describes filtration in volume/s (<math>J_\mathrm{v}</math>) by relating oncotic pressure (<math>\pi_\mathrm{p}</math>) to capillary hydrostatic pressure (<math>P_\mathrm{c}</math>), interstitial fluid hydrostatic pressure (<math>P_\mathrm{i}</math>), and interstitial fluid oncotic pressure (<math>\pi_\mathrm{i}</math>), as well as several descriptive coefficients, as shown below:

<math>\ J_\mathrm{v} = L_\mathrm{p} S ( [P_\mathrm{c} - P_\mathrm{i}] - \sigma[\pi_\mathrm{p} - \pi_\mathrm{i}] )</math>

At the arteriolar end of the capillary, blood pressure starts at about 36&nbsp;mm Hg and decreases to around 15&nbsp;mm Hg at the venous end, with oncotic pressure at a stable 25–28&nbsp;mm Hg. Within the capillary, reabsorption due to this venous pressure difference is estimated to be around 90% that of the filtered fluid, with the extra 10% being returned via lymphatics in order to maintain stable blood volume.<ref>{{Citation|last1=Darwish|first1=Alex|title=Physiology, Colloid Osmotic Pressure|date=2021|url=http://www.ncbi.nlm.nih.gov/books/NBK541067/|work=StatPearls|place=Treasure Island (FL)|publisher=StatPearls Publishing|pmid=31082111|access-date=2021-12-09|last2=Lui|first2=Forshing}}</ref>