Editing Capillary (section)

== Structure ==
[[File:A red blood cell in a capillary, pancreatic tissue - TEM.jpg|thumb|left|[[Transmission electron microscope]] image of a cross-section of a capillary occupied by a [[red blood cell]]]]

Blood flows from the heart through [[arteries]], which branch and narrow into [[arteriole]]s, and then branch further into capillaries where nutrients and wastes are exchanged. The capillaries then join and widen to become [[venule]]s, which in turn widen and converge to become [[vein]]s, which then return blood back to the heart through the [[venae cavae]]. In the [[mesentery]], [[metarteriole]]s form an additional stage between arterioles and capillaries.

Individual capillaries are part of the '''capillary bed''', an interweaving network of capillaries supplying [[Tissue (biology)|tissue]]s and [[organ (biology)|organs]]. The more [[metabolically]] active a tissue is, the more capillaries are required to supply nutrients and carry away products of metabolism. There are two types of capillaries: true capillaries, which branch from arterioles and provide exchange between tissue and the capillary blood, and [[#Sinusoidal|sinusoids]], a type of open-pore capillary found in the [[liver]], [[bone marrow]], [[anterior pituitary gland]], and brain [[circumventricular organs]]. Capillaries and sinusoids are short vessels that directly connect the arterioles and venules at opposite ends of the beds. [[Metarteriole]]s are found primarily in the [[mesenteric]] [[microcirculation]].<ref name="Sakai 13">{{cite journal | pmc=3751330 | title=Are the precapillary sphincters and metarterioles universal components of the microcirculation? An historical review  | journal=The Journal of Physiological Sciences | year=2013 | pmid=23824465 | doi=10.1007/s12576-013-0274-7 | volume=63 | issue=5 | pages=319–31| last1=Sakai  | first1=T  | last2=Hosoyamada  | first2=Y  }}</ref>

[[Lymphatic capillaries]] are slightly larger in diameter than blood capillaries, and have closed ends (unlike the blood capillaries open at one end to the arterioles and open at the other end to the venules). This structure permits [[interstitial fluid]] to flow into them but not out. Lymph capillaries have a greater internal [[oncotic pressure]] than blood capillaries, due to the greater concentration of [[plasma proteins]] in the [[lymph]].<ref>{{cite book |last1=Guyton |first1=Arthur C. |last2=Hall |first2=John Edward |title=Textbook of Medical Physiology |date=2006 |publisher=Elsevier Saunders |location=Philadelphia |isbn=9780808923176 |pages=187–188 |edition=11th |chapter=The Microcirculation and the Lymphatic System}}</ref>

=== Types ===
[[File:Different Types of Capillaries.jpg|thumb|upright=1.2|Types of capillaries: (left) continuous with no big gaps, (center) fenestrated with small pores, and (right) sinusoidal (or 'discontinuous') with intercellular gaps]]

Blood capillaries are categorized into three types: continuous, fenestrated, and sinusoidal (also known as discontinuous).

==== Continuous ====
Continuous capillaries are continuous in the sense that the endothelial cells provide an uninterrupted lining, and they only allow smaller [[molecule]]s, such as water and [[ion]]s, to pass through their [[intercellular cleft]]s.<ref name="keep">{{cite journal |pmc=4836471|year=2016|last1=Stamatovic|first1=S. M.|title=Junctional proteins of the blood-brain barrier: New insights into function and dysfunction|journal=Tissue Barriers|volume=4|issue=1|pages=e1154641|last2=Johnson|first2=A. M.|last3=Keep|first3=R. F.|last4=Andjelkovic|first4=A. V.|pmid=27141427|doi=10.1080/21688370.2016.1154641}}</ref><ref name="wilhelm">{{cite journal|pmc=4836475|year= 2016|last1=Wilhelm|first1=I.|title=Heterogeneity of the blood-brain barrier|journal=Tissue Barriers|volume=4|issue=1|pages=e1143544|last2=Suciu|first2=M.|last3=Hermenean|first3=A.|last4=Krizbai|first4=I. A.|pmid=27141424|doi= 10.1080/21688370.2016.1143544}}</ref> Lipid-soluble molecules can passively diffuse through the endothelial cell membranes along concentration gradients.<ref name="sarin">{{cite journal |pmc=3234205|year=2010|last1=Sarin|first1= H.|title=Overcoming the challenges in the effective delivery of chemotherapies to CNS solid tumors|journal=Therapeutic Delivery|volume=1|issue=2|pages=289–305|doi=10.4155/tde.10.22|pmid=22163071}}</ref> Continuous capillaries can be further divided into two subtypes:
:# Those with numerous transport vesicles, which are found primarily in [[skeletal muscle]]s, fingers, [[gonad]]s, and skin.<ref>{{cite journal |pmid=22574942|year=2012|last1=Michel|first1=C. C.|title=Electron tomography of vesicles|journal=Microcirculation |volume=19|issue=6|pages=473–6|doi=10.1111/j.1549-8719.2012.00191.x|s2cid=205759387|doi-access=free}}</ref>
:# Those with few vesicles, which are primarily found in the [[central nervous system]]. These capillaries are a constituent of the [[blood–brain barrier]].<ref name=wilhelm/>

==== Fenestrated ====
Fenestrated capillaries have pores known as [[fenestra (histology)|''fenestrae'']] ([[Latin]] for "windows") in the endothelial cells that are 60–80&nbsp;[[nanometre]]s (nm) in diameter. They are spanned by a diaphragm of radially oriented [[fibril]]s that allows small molecules and limited amounts of protein to diffuse.<ref>{{BUHistology|22401lba|inline=1}}</ref><ref>{{cite book |last1=Pavelka |first1=Margit |last2=Roth |first2=Jürgen |title= Functional Ultrastructure: An Atlas of Tissue Biology and Pathology |date=2005 |publisher=Springer |location=Vienna |isbn=978-3-211-26392-1 |page=232 |url=https://doi.org/10.1007/3-211-26392-6_120 |language=en |chapter=Fenestrated Capillary|doi=10.1007/3-211-26392-6_120 }}</ref> In the [[renal glomerulus]] the capillaries are wrapped in [[podocyte foot processes]] or pedicels, which have slit pores with a function analogous to the diaphragm of the capillaries. Both of these types of blood vessels have continuous [[basal lamina]]e and are primarily located in the [[endocrine gland]]s, [[intestines]], [[pancreas]], and the [[renal glomerulus|glomeruli]] of the [[kidney]].

==== Sinusoidal ====
[[File:Sinusoid.jpeg|alt=Scanning electron micrograph of a liver sinusoid with fenestrated endothelial cells.|thumb|[[Scanning electron micrograph]] of a [[liver sinusoid]] with fenestrated endothelial cells. Fenestrae are approximately 100&nbsp;nm in diameter.]]

Sinusoidal capillaries or discontinuous capillaries are a special type of open-pore capillary, also known as a ''sinusoid'',<ref name="HLM">{{cite web |title=Histology Laboratory Manual |url= http://www.columbia.edu/itc/hs/medical/sbpm_histology_old/lab/lab07_micrograph.html |website=www.columbia.edu}}</ref> that have wider fenestrations that are 30–40&nbsp;[[micrometre]]s (μm) in diameter, with wider openings in the endothelium.<ref name="Saladin">{{Cite book| isbn=9780071222075|title=Human Anatomy|last1=Saladin|first1=Kenneth S.|year=2011|pages=568–569|publisher=McGraw-Hill }}</ref> Fenestrated capillaries have diaphragms that cover the pores whereas sinusoids lack a diaphragm and just have an open pore. These types of blood vessels allow [[red blood cell|red]] and [[white blood cell]]s (7.5&nbsp;μm – 25&nbsp;μm diameter) and various [[Blood plasma|serum]] proteins to pass, aided by a discontinuous basal lamina. These capillaries lack [[pinocytotic vesicles]], and therefore use gaps present in cell junctions to permit transfer between endothelial cells, and hence across the membrane. Sinusoids are irregular spaces filled with blood and are mainly found in the [[liver]], [[bone marrow]], [[spleen]], and brain [[circumventricular organs]].<ref name="Saladin"/><ref name="gross1">{{cite book|pmid=1410407|year=1992|last1=Gross|first1=P. M|chapter=Chapter 31: Circumventricular organ capillaries |title=Circumventricular Organs and Brain Fluid Environment - Molecular and Functional Aspects|series=Progress in Brain Research|volume=91|pages=219–33|doi=10.1016/S0079-6123(08)62338-9|isbn=9780444814197}}</ref>