Editing Capillary (section)

== Function ==
{{See also|Microcirculation#Capillary exchange}}
[[File:The exchange between capillary and body tissue diagram.svg|thumb|Annotated diagram of the exchange between capillary and body tissue through the exchange of materials between cells and fluid]]
The capillary wall performs an important function by allowing nutrients and waste substances to pass across it. Molecules larger than 3&nbsp;nm such as [[albumin]] and other large proteins pass through [[transcellular transport]] carried inside [[Vesicle (biology and chemistry)|vesicles]], a process which requires them to go through the cells that form the wall. Molecules smaller than 3&nbsp;nm such as water and gases cross the capillary wall through the space between cells in a process known as [[paracellular transport]].<ref name=sukriti>{{cite journal|last1=Sukriti|first1=S|last2=Tauseef|first2=M|last3=Yazbeck|first3=P|last4=Mehta|first4=D|title=Mechanisms regulating endothelial permeability|journal=Pulmonary Circulation|year= 2014|volume=4|issue=4|pages=535–551|pmid=25610592|pmc=4278616|doi=10.1086/677356}}</ref> These transport mechanisms allow bidirectional exchange of substances depending on [[osmosis|osmotic]] gradients.<ref name=nagy>{{cite journal|last1=Nagy|first1=JA|last2=Benjamin|first2=L|last3=Zeng|first3=H|last4=Dvorak|first4=AM|last5=Dvorak|first5= HF|title=Vascular permeability, vascular hyperpermeability and angiogenesis|journal=Angiogenesis|year=2008|volume=11|issue=2|pages=109–119|doi=10.1007/s10456-008-9099-z|pmid=18293091|pmc=2480489}}</ref> Capillaries that form part of the [[blood–brain barrier]] only allow for transcellular transport as [[tight junction]]s between endothelial cells seal the paracellular space.<ref name=bauer>{{cite journal|last1=Bauer|first1=HC|last2= Krizbai|first2=IA|last3=Bauer|first3=H|last4=Traweger|first4=A|year=2014|title="You Shall Not Pass"-tight junctions of the blood brain barrier|journal=Frontiers in Neuroscience|volume=8|pages=392|pmid=25520612|pmc=4253952|doi= 10.3389/fnins.2014.00392|doi-access=free }}</ref>

Capillary beds may control their blood flow via [[autoregulation]]. This allows an organ to maintain constant flow despite a change in central blood pressure. This is achieved by [[myogenic response]], and in the [[kidney]] by [[tubuloglomerular feedback]]. When blood pressure increases, arterioles are stretched and subsequently constrict (a phenomenon known as the [[Bayliss effect]]) to counteract the increased tendency for high pressure to increase blood flow.<ref>{{cite book |last1=Boulpaep |first1=Emile L. |editor1-last=Boron |editor1-first=Walter F. |editor2-last=Boulpaep |editor2-first=Emile L. |title=Medical Physiology |date=2017 |publisher=Elsevier |location=Philadelphia, PA |isbn=978-1-4557-4377-3 |page=481 |edition=3rd |chapter=The Microcirculation}}</ref>

In the [[lung]]s, special mechanisms have been adapted to meet the needs of increased necessity of blood flow during exercise. When the [[heart rate]] increases and more blood must flow through the lungs, capillaries are recruited and are also distended to make room for increased blood flow. This allows blood flow to increase while resistance decreases.{{cn|date=January 2015}} Extreme exercise can make capillaries vulnerable, with a breaking point similar to that of [[collagen]].<ref>{{Cite journal |last=West |first=J. B. |date=2006 |title=Vulnerability of pulmonary capillaries during severe exercise |journal=[[British Journal of Sports Medicine]] |volume=40 |issue=10 |pages=821 |doi=10.1136/bjsm.2006.028886 |issn=1473-0480 |pmc=2465077 |pmid=17021008}}</ref>

Capillary [[vascular permeability|permeability]] can be increased by the release of certain [[cytokine]]s, [[anaphylatoxin]]s, or other mediators (such as leukotrienes, prostaglandins, histamine, bradykinin, etc.) highly influenced by the [[immune system]].<ref>{{Citation |last1=Yunfei |first1=Chi |title=A narrative review of changes in microvascular permeability after burn |date=2021-04-09 |editor-last=Jiake |editor-first=Chai |last2=Xiangyu |first2=Liu|journal=Annals of Translational Medicine |volume=9 |issue=8 |page=719 |doi=10.21037/atm-21-1267 |doi-access=free |pmid=33987417 |pmc=8106041 }}</ref>

=== Starling equation ===
[[File:2108 Capillary Exchange.jpg|thumb|upright=1.6|Diagram of the filtration and reabsorption in capillaries]]
The transport mechanisms can be further quantified by the [[Starling equation]].<ref name=nagy/> The Starling equation defines the forces across a semipermeable membrane and allows calculation of the net flux:

:<math>J_v = K_f [(P_c - P_i) - \sigma(\pi_c - \pi_i)],</math>

where:

: <math> (P_c - P_i) - \sigma(\pi_c - \pi_i) </math> is the net driving force,
: <math> K_f </math> is the proportionality constant, and
: <math> J_v </math> is the net fluid movement between compartments.

By convention, outward force is defined as positive, and inward force is defined as negative. The solution to the equation is known as the net filtration or net fluid movement (''J''<sub>''v''</sub>). If positive, fluid will tend to ''leave'' the capillary (filtration). If negative, fluid will tend to ''enter'' the capillary (absorption). This equation has a number of important physiologic implications, especially when pathologic processes grossly alter one or more of the variables.{{cn|date=January 2015}}

According to Starling's equation, the movement of fluid depends on six variables:
# Capillary [[hydrostatic pressure]] (''P''<sub>''c''</sub>)
# Interstitial hydrostatic pressure (''P''<sub>''i''</sub>)
# Capillary [[oncotic pressure]] ({{pi}}<sub>''c''</sub>)
# Interstitial oncotic pressure ({{pi}}<sub>''i''</sub>)
# Filtration coefficient (''K''<sub>''f''</sub>)
# Reflection coefficient (''σ'')