Editing Microcirculation

{{Short description|Circulation of the blood in the smallest blood vessels}}
{{Infobox anatomy
| Name        = Microcirculation
| Latin       =
| Image       = Capillary microcirculation.svg
| Caption     = Microcirculation in the capillary
| Width       =
| Image2      =
| Caption2    =
| Precursor   =
| System      = [[Circulatory system]]
| Artery      = [[Arteriole]]
| Vein        = [[Venule]]
| Nerve       =
| Lymph       =
}}

The '''microcirculation''' is the [[circulatory system|circulation]] of the [[blood]] in the smallest [[blood vessel]]s, the [[microvessel]]s of the '''microvasculature''' present within [[organ (anatomy)|organ]] [[Tissue (biology)|tissues]].<ref name=Conti>{{cite book |last1=Conti |first1=Fiorenzo  |title=Fisiología Médica |year= 2010 |edition=1st |publisher=Mc-Graw Hill |isbn=978-970-10-7341-4}}{{page needed|date=May 2015}}</ref> The microvessels include terminal [[arteriole]]s, [[metarteriole]]s, [[capillaries]], and [[venule]]s.  Arterioles carry oxygenated blood to the capillaries, and blood flows out of the capillaries through venules into [[veins]].{{cn|date=June 2022}}

In addition to these blood vessels, the microcirculation also includes [[lymphatic capillaries]] and collecting ducts. The main functions of the microcirculation are the delivery of [[oxygen]] and nutrients and the removal of [[carbon dioxide]] (CO<sub>2</sub>). It also serves to regulate blood flow and tissue perfusion, thereby affecting blood pressure and responses to [[inflammation]] which can include [[edema]] (swelling).

Most vessels of the microcirculation are lined by flattened cells of the [[endothelium]] and many of them are surrounded by contractile cells called [[pericytes]]. The endothelium provides a smooth surface for the flow of blood and regulates the movement of water and dissolved materials in the interstitial plasma between the blood and the tissues.

The microcirculation contrasts with '''macrocirculation''', which is the circulation of blood to and from the organs.

==Structure==
===Microvessels===
[[File:2105 Capillary Bed.jpg| thumb | right | Blood flows away from the heart to [[artery|arteries]], which follow into [[arteriole]]s, and then narrow further into capillaries.  After the tissue has been [[perfusion|perfused]], capillaries branch and widen to become [[venule]]s and then widen more and connect to become [[vein]]s, which return blood to the heart.]]
[[File:A red blood cell in a capillary, pancreatic tissue - TEM.jpg|thumb|Transmission electron microscope image of a capillary with a red blood cell within the pancreas. The capillary lining consists of long, thin endothelial cells, connected by [[tight junction]]s.]]
The vessels on the arterial side of the microcirculation are called the [[arteriole]]s, which are well innervated, are surrounded by [[smooth muscle]] cells, and are 10-50&nbsp;[[micrometre|μm]] in diameter.<ref name="Formaggia">{{cite book |last1=Formaggia |first1=Luca |author-link2= Alfio Quarteroni|last2=Quarteroni |first2=Alfio |last3=Veneziani |first3=Alessandro |title=Cardiovascular mathematics: modeling and simulation of the circulatory system |date=2009 |isbn=8847011515 |page=6 |url=https://link.springer.com/book/10.1007/978-88-470-1152-6 |access-date=1 March 2023}}</ref> Arterioles carry the blood to the [[Capillary|capillaries]], which are not innervated, have no smooth muscle, and are about 5–8&nbsp;μm in diameter.  Blood flows out of the capillaries into the [[venule]]s, which have little smooth muscle and are 10–200&nbsp;μm.  The blood flows from the venules into the [[veins]]. [[Metarteriole]]s connect arterioles and capillaries. A tributary to the venules is known as a ''thoroughfare channel''.{{citation needed|date=November 2021}}

The microcirculation has three major components: pre-capillary, capillary, and post-capillary. In the pre-capillary sector, arterioles, and [[precapillary sphincter]]s participate. Their function is to regulate blood flow before it enters the capillaries and venules by the contraction and relaxation of the smooth muscle found on their walls. The second sector is the capillary sector, which is represented by the capillaries, where substance and gas exchange between blood and interstitial fluid takes place. Finally, the post-capillary sector is represented by the post-capillary venules, which are formed by a layer of [[endothelial cells]] that allow free movement of some substances.<ref name="Drucker">{{cite book |last1=Drucker |first1=René |title=Medical physiology |edition=1st |publisher=Modern Manual |page=137}}</ref>

=== Microanatomy ===
Most vessels of the microcirculation are lined by flattened cells of the [[endothelium]] and many of them are surrounded by contractile cells called [[pericytes]]. The endothelium provides a smooth surface for the flow of blood and regulates the movement of water and dissolved materials in the interstitial plasma between the blood and the tissues.  The endothelium also produces molecules that discourage the blood from clotting unless there is a leak.  Pericyte cells can contract and decrease the size of the arterioles and thereby regulate blood flow and blood pressure.{{citation needed|date=November 2021}}

== Function ==
In addition to these blood vessels, the microcirculation also includes [[lymphatic capillaries]] and collecting ducts. The main functions of the microcirculation are the delivery of [[oxygen]] and nutrients and the removal of [[carbon dioxide]] (CO<sub>2</sub>). It also serves to regulate blood flow and tissue perfusion thereby affecting blood pressure and responses to [[inflammation]] which can include [[edema]] (swelling).{{citation needed|date=November 2021}}

===Regulation===
The regulation of tissue [[perfusion]] occurs in microcirculation.<ref name=Drucker/> There, [[arterioles]] control the flow of blood to the capillaries. Arterioles contract and relax, varying their diameter and vascular tone, as the vascular smooth muscle responds to diverse stimuli. Distension of the vessels due to increased blood pressure is a fundamental stimulus for muscle contraction in arteriolar walls. As a consequence, microcirculation blood flow remains constant despite changes in systemic blood pressure. This mechanism is present in all tissues and organs of the human body. In addition, the nervous system participates in the regulation of microcirculation. The sympathetic nervous system activates the smaller arterioles, including terminals. [[Noradrenaline]] and [[adrenaline]] have effects on alpha and beta adrenergic receptors. Other hormones ([[catecholamine]], [[Renin–angiotensin system|renin-angiotensin]], [[vasopressin]], and [[atrial natriuretic peptide]]) circulate in the bloodstream and can have an effect on the microcirculation causing [[vasodilation]] or [[vasoconstriction]]. Many hormones and neuropeptides are released together with classical neurotransmitters.<ref name=Conti/>

Arterioles respond to metabolic stimuli that are generated in the tissues. When tissue metabolism increases, [[catabolic]] products accumulate leading to vasodilation. The endothelium begins to control muscle tone and arteriolar blood flow tissue. Endothelial function in the circulation includes the activation and inactivation of circulating hormones and other plasma constituents. There are also synthesis and secretion of vasodilator and vasoconstrictor substances for modifying the width as necessary. Variations in the flow of blood that circulates by arterioles are capable of responses in endothelium.<ref name=Conti/>

===Capillary exchange===
The term capillary exchange refers to all exchanges at microcirculatory level, most of which occurs in the capillaries. Sites where material exchange occurs between the blood and tissues are the capillaries, which branch out to increase the swap area, minimize the diffusion distance as well as maximize the surface area and the exchange time.<ref name=Sherwood>{{cite book |last1=Sherwood |first1=Lauralee |title=Human Physiology. From cells to systems. |year=2005 |edition=7th |publisher=Cengage learning |isbn=970-729-069-2 |page=361}}</ref>

Approximately, seven percent of the body's blood is in the capillaries which continuously exchange substances with the liquid outside these blood vessels, called interstitial fluid. This dynamic displacement of materials between the interstitial fluid and the blood is named capillary exchange.<ref name=Tortora>{{cite book |last1=Tortora |first1=Gerard |title=Principles of anatomy and physiology |date=2011 |edition=13th |publisher=Wiley & Sons, Inc. |isbn=978-0470565100 |page=811}}</ref> These substances pass through capillaries through three different systems or mechanisms: diffusion, bulk flow, and transcytosis or vesicular transport.<ref name=Drucker/> The liquid and solid exchanges that take place in the microvasculature particularly involve capillaries and post-capillary venules and collecting venules.{{citation needed|date=November 2021}}

Capillary walls allow the free flow of almost every substance in plasma.<ref name=Guyton>{{cite book |last1=Hall |first1=John |title=Textbook of Medical Physiology |year=2011 |edition=12th |publisher=Elsevier Science Publishers |isbn=978-84-8086-819-8 |page=184}}</ref> The plasma proteins are the only exception, as they are too big to pass through.<ref name=Tortora/> The minimum number of un-absorbable plasma proteins that exit capillaries enter lymphatic circulation for returning later on to those blood vessels. Those proteins which leave capillaries use the first capillary exchange mechanism and the process of diffusion, which is caused by kinetic motion of molecules.<ref name=Guyton/>

====Regulation====
These exchanges of substances are regulated by different mechanisms.<ref name=Klaubunde>{{cite book |last1=Klaubunde |first1=Richard |title=Cardiovascular physiology concepts |date= 2011 |edition=2nd |publisher=Lippincott Williams & Wilkins |isbn=9781451113846 |page=181}}</ref> These mechanisms work together and promote capillary exchange in the following way. First, molecules that diffuse are going to travel a short distance thanks to the capillary wall, the small diameter and the close proximity to each cell having a capillary. The short distance is important because the capillary diffusion rate decreases when the diffusion distance increases. Then, because of its large number (10–14 million capillaries), there is an incredible amount of surface area for exchange. However, this only has 5% of the total blood volume (250 ml/5000 ml). Finally, blood flows more slowly in the capillaries, given the extensive branching.<ref name="Sherwood"/>

====Diffusion====
[[Diffusion]] is the first and most important mechanism that allows the flow of small molecules across capillaries. The process depends on the difference of gradients between the [[interstitium]] and blood, with molecules moving to low concentrated spaces from high concentrated ones.<ref name=Johnson>{{cite book |last1=Johnson |first1=Leonard |title= Essential medical physiology|date=2003 |edition=3rd |publisher= Academic Press |isbn=978-0123875846 |page=59}}</ref> Glucose, amino acids, oxygen ({{O2}}) and other molecules exit capillaries by diffusion to reach the organism's tissues. Contrarily, carbon dioxide ({{CO2}}) and other wastes leave tissues and enter capillaries by the same process but in reverse.<ref name=Tortora/> Diffusion through the capillary walls depends on the permeability of the endothelial cells forming the capillary walls, which may be continuous, discontinuous, and fenestrated.<ref name=Sherwood/> The [[Starling equation]] describes the roles of [[hydrostatic pressure|hydrostatic]] and [[osmotic pressure]]s  (the so-called '''Starling forces''') in the movement of fluid across capillary [[endothelium]]. Lipids, which are transported by proteins, are too large to cross the capillary walls by diffusion, and have to rely on the other two methods.<ref>{{cite journal |pmid=6995154 |year=1980 |last1=Scow |first1=R.O |title=Transport of lipid across capillary endothelium |journal=Federation Proceedings |volume=39 |issue=9 |pages=2610–17 |last2=Blanchette-Mackie |first2=E.J |last3=Smith |first3=L.C }}</ref><ref>{{Cite web | url=https://www.anaesthesiamcq.com/FluidBook/fl4_1.php | title=Fluid Physiology: 4.1 Microcirculation}}</ref>

====Bulk flow====
The second mechanism of capillary exchange is [[Mass flow (life sciences)|bulk flow]]. It is used by small, lipid-insoluble substances in order to cross. This movement depends on the physical characteristics of the capillaries. For example, continuous capillaries (tight structure) reduce bulk flow, [[Fenestra (histology)|fenestrated]] capillaries (perforated structure) increases bulk flow, and discontinuous capillaries (great intercellular gaps) enable bulk flow. In this case, the exchange of materials is determined by changes in pressure.<ref name=Klaubunde/> When the flow of substances goes from the bloodstream or the capillary to the interstitial space or interstitium, the process is called filtration. This kind of movement is favored by blood hydrostatic pressure (BHP) and interstitial fluid osmotic pressure (IFOP).<ref name=Tortora/> When substances move from the interstitial fluid to the blood in capillaries, the process is called reabsorption. The pressures that favor this movement are blood colloid osmotic pressure (BCOP) and interstitial fluid hydrostatic pressure (IFHP).<ref name=Scallan>{{cite book |last1=Scallan |first1=Joshua |title=Capillary Fluid Exchange: Regulation, Functions and Pathology |year=2010 |edition=3rd |publisher=Morgan & Claypool Life Sciences. |isbn=9781615040667 |page= 4}}</ref> Whether a substance is filtrated or reabsorbed depends on the net filtration pressure (NFP), which is the difference between hydrostatic (BHP and IFHP) and osmotic pressures (IFOP and BCOP).<ref name=Tortora/> These pressures are known as the [[Starling forces]]. If the NFP is positive then there will be filtration, but if it is negative then reabsorption will occur.<ref name=Sicar>{{cite book |last1=Sicar |first1= Sabyasachi |title= Principles of medical physiology. |year= 2008 |edition=1st |publisher= Lippincott Williams & Wilkins |isbn=978-3-13-144061-7 |page=259}}</ref>

====Transcytosis====
The third capillary exchange mechanism is [[transcytosis]], also called vesicular transport.<ref name=Barret>{{cite book |last1=Barret |first1=Kim |title=Ganong Medical Physiology |year= 2012 |edition=24th |publisher=Mc-Graw Hill |isbn=978-0071780032 }}</ref> By this process, blood substances move across the endothelial cells that compose the capillary structure. Finally, these materials exit by exocytosis, the process by which vesicles go out from a cell to the interstitial space. Few substances cross by transcytosis: it is mainly used by large, lipid-insoluble molecules such as the insulin hormone.<ref name=Shahid>{{cite book |last1=Shahid |first1=Mohammad |title=Physiology |date=2008 |edition=1st |publisher=Elsevier Health Sciences |isbn=978-0-7234-3388-0 |page=82}}</ref> Once vesicles exit the capillaries, they go to the [[interstitium]].<ref name="Shahid"/> Vesicles can go directly to a specific tissue or they can merge with other vesicles, so their contents are mixed. This intermixed material increases the functional capability of the vesicle.<ref name=Tortora/>

== See also ==
* [[Fahraeus–Lindquist effect]]
* [[Glycocalyx]]
* [[Microcirculatory Society]]

==References==
{{Reflist}}

{{Commons category|Microcirculation}}

{{Arteries and veins}}
{{Authority control}}

[[Category:Circulatory system]]