Editing Oncotic pressure

{{Short description|Measure of pressure exerted by large dissolved molecules in biological fluids}}
[[File:Colloid Osmotic Pressure.png|thumb|Above, we see a representation of fluid flow in the presence of colloids, with the left side representing surrounding tissues and the right representing whole blood. The presence of colloids can increase the flow towards the high concentration of colloids by creating colloid osmotic pressure in an otherwise state of equilibrium.]]
[[File:Onc Pressure.png|thumb|In the illustration above, we see how the osmotic pressure changes over the length of the capillary, with oncotic pressure remaining the same. Overall direction of fluid flow in relation to equal bidirectional flow is shown by the orange and black lines, respectively.]]

'''Oncotic pressure''', or '''colloid osmotic-pressure''', is a type of [[osmotic pressure]] induced by the [[plasma protein]]s, notably [[albumin]],<ref>{{Citation|last1=Moman|first1=Rajat N.|title=Physiology, Albumin|date=2021|url=http://www.ncbi.nlm.nih.gov/books/NBK459198/|work=StatPearls|place=Treasure Island (FL)|publisher=StatPearls Publishing|pmid=29083605|access-date=2021-12-09|last2=Gupta|first2=Nishant|last3=Varacallo|first3=Matthew}}</ref> in a blood vessel's plasma (or any other [[body fluid]] such as [[blood]] and [[lymph]]) that causes a pull on fluid back into the capillary.

It has an effect opposing both the [[hydrostatic]] [[blood pressure]], which pushes water and small molecules out of the blood into the interstitial spaces at the arterial end of capillaries, and the [[extracellular fluid|interstitial]] colloidal osmotic pressure. These interacting factors determine the partitioning of [[extracellular fluid|extracellular water]] between the blood plasma and the extravascular space.

Oncotic pressure strongly affects the physiological function of the circulatory system. It is suspected to have a major effect on the pressure across the [[glomerulus (kidney)|glomerular]] filter. However, this concept has been strongly criticised and attention has shifted to the impact of the intravascular glycocalyx layer as the major player.<ref>{{cite journal | vauthors = Levick JR, Michel CC | title = Microvascular fluid exchange and the revised Starling principle | journal = Cardiovascular Research | volume = 87 | issue = 2 | pages = 198–210 | date = July 2010 | pmid = 20200043 | doi = 10.1093/cvr/cvq062 | doi-access = free }}</ref><ref>{{cite journal | vauthors = Raghunathan K, Murray PT, Beattie WS, Lobo DN, Myburgh J, Sladen R, Kellum JA, Mythen MG, Shaw AD | display-authors = 6 | title = Choice of fluid in acute illness: what should be given? An international consensus | journal = British Journal of Anaesthesia | volume = 113 | issue = 5 | pages = 772–83 | date = November 2014 | pmid = 25326478 | doi = 10.1093/bja/aeu301 | doi-access = free }}</ref><ref>{{cite journal | vauthors = Woodcock TE, Woodcock TM | title = Revised Starling equation and the glycocalyx model of transvascular fluid exchange: an improved paradigm for prescribing intravenous fluid therapy | journal = British Journal of Anaesthesia | volume = 108 | issue = 3 | pages = 384–94 | date = March 2012 | pmid = 22290457 | doi = 10.1093/bja/aer515 | doi-access = free }}</ref><ref>{{cite book|last1=Maitra|first1=Sayantan|chapter=Chapter 18 - Salt-induced inappropriate augmentation of renin–angiotensin–aldosterone system in chronic kidney disease|date=2020-01-01|chapter-url=https://www.sciencedirect.com/science/article/pii/B9780128169186000184|title=Dietary Sugar, Salt and Fat in Human Health|pages=377–393|editor-last=Preuss|editor-first=Harry G.|publisher=Academic Press|language=en|isbn=978-0-12-816918-6|access-date=2021-12-10|last2=Dutta|first2=Dibyendu|editor2-last=Bagchi|editor2-first=Debasis}}</ref>

== Etymology ==
The word 'oncotic' by definition is termed as 'pertaining to swelling', indicating the effect of oncotic imbalance on the swelling of tissues.

The word itself is derived from onco- and -ic; 'onco-' meaning 'pertaining to mass or tumors' and '-ic', which forms an adjective.

==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>

==Physiological impact==
In tissues, physiological disruption can arise with decreased oncotic pressure, which can be determined using blood tests for protein concentration.

Decreased colloidal osmotic pressure, most notably seen in [[hypoalbuminemia]], can cause edema and decrease in blood volume as fluid is not reabsorbed into the bloodstream. Colloid pressure in these cases can be lost due to a number of different factors, but primarily decreased colloid production or increased loss of colloids through glomerular filtration.<ref name=":0" /><ref>{{Citation|last1=Prasad|first1=Rohan M.|title=Microalbuminuria|date=2021|url=http://www.ncbi.nlm.nih.gov/books/NBK563255/|work=StatPearls|place=Treasure Island (FL)|publisher=StatPearls Publishing|pmid=33085402|access-date=2021-12-09|last2=Tikaria|first2=Richa}}</ref> This low pressure often correlates with poor surgical outcomes.<ref>{{Cite journal|last1=Kim|first1=Sunghye|last2=McClave|first2=Stephen A.|last3=Martindale|first3=Robert G.|last4=Miller|first4=Keith R.|last5=Hurt|first5=Ryan T.|date=2017-11-01|title=Hypoalbuminemia and Clinical Outcomes: What is the Mechanism behind the Relationship?|journal=The American Surgeon|volume=83|issue=11|pages=1220–1227|doi=10.1177/000313481708301123|issn=1555-9823|pmid=29183523|s2cid=25304336 |doi-access=free}}</ref>

In the clinical setting, there are two types of fluids that are used for intravenous drips: crystalloids and [[colloid]]s. Crystalloids are [[aqueous solution]]s of mineral salts or other water-soluble molecules. Colloids contain larger insoluble molecules, such as [[gelatin]]. There is some debate concerning the advantages and disadvantages of using biological vs. synthetic colloid solutions.<ref>{{Cite journal|last1=Wong|first1=Christine|last2=Koenig|first2=Amie|date=March 2017|title=The Colloid Controversy: Are Colloids Bad and What Are the Options?|url=https://pubmed.ncbi.nlm.nih.gov/27914756/|journal=The Veterinary Clinics of North America. Small Animal Practice|volume=47|issue=2|pages=411–421|doi=10.1016/j.cvsm.2016.09.008|issn=1878-1306|pmid=27914756}}</ref> Oncotic pressure values are approximately 290 mOsm per kg of water, which slightly differs from the osmotic pressure of the blood that has values approximating 300&nbsp;mOsm /L.{{citation needed|date=June 2014}} These colloidal solutions are typically used to remedy low colloid concentration, such as in hypoalbuminemia, but is also suspected to assist in injuries that typically increase fluid loss, such as burns.<ref>{{Cite journal|last1=Cartotto|first1=Robert|last2=Greenhalgh|first2=David|date=October 2016|title=Colloids in Acute Burn Resuscitation|url=https://pubmed.ncbi.nlm.nih.gov/27600123/|journal=Critical Care Clinics|volume=32|issue=4|pages=507–523|doi=10.1016/j.ccc.2016.06.002|issn=1557-8232|pmid=27600123}}</ref>

== References ==
{{reflist}}

== External links ==
* [http://www.cvphysiology.com/Microcirculation/M012.htm Overview at cvphysiology.com]

[[Category:Physiology]]